Eagle

Yes. I use a lot of Crown parts. They specialize in Jeep stuff (along with Hudson!). So much so that their Jeep parts use Jeep part numbers. Ratio for the XJ/MJ manual box was either 20:1 or 24:1. VERY slow ratio, and I can't imagine actually wanting to replace one with another of the same. That said, it's a basic Saginaw box, and the same box was used in AMC cars for decades. Go junkyard, or splurge on a rebuilt.

First step is to determine if the water causing the rust attacked from the underside or the inside (hidden leak).

Something wrong? Yes ... you're buying Rusty's junk. I made the mistake of buying a Rusty's adjustable track bar. I've had the old factory track bars, the ones with no grease fitting on the frame end TRE, last over 100,000 miles. The Rusty's adjustable lasted less than 10,000. Fortunately, on an adjustable the TRE is replaceable. One of my friends in NAXJA NAC, however, had the threaded insert at the upper end of the bar rip out of the tube. You can't fix that with a new TRE from NAPA. I really don't understand how Rusty's stays in business -- or how he sleeps at night.

Correct -- assuming it's all original. No. There are no aftermarket 15x8 wheels that have as much backspacing as factory Jeep wheels. Unless you get a set of factory Jeep 15x8s, the wheels will take care of any spacing issues. What you will have to look out for is the shoulder of the front tire hitting the lower back corner of the wheel arch when turning. Acceleration will be worse, and 5th gear will be nearly useless. I ran my '88 on 31s for several years with 3.73 gears. In terms of speed-to-RPMs, this combination is exactly equal to stock tires with an automatic and 3.55 gears. Very nice for street, but 4.10s would have been better for off-road. I am currently running 31s on my '88 XJ with the stock 3.07 gears. I've been driving manual transmissions for well over 50 years so I'm not likely to burn out the clutch, but fifth gear is only useful on highways with a speed limit of 65 or higher. With 3.55s, you'll be somewhere in between my two examples. A new speedometer gear. What don't you understand? Find the chart for the tire size. Find the pair of columns with your gear ratio, and each row tells you the RPMs your engine will turn in 4th and 5th gear for the road speeds listed on the left. For your situation, 31" tires and 3.55 gears, at 65 MPH the engine would turn 1956 RPM in 5th gear, and 2608 RPM in 4th gear. That's not a horrible setup.In fact, a stock 4.0L 5-speed on 225/75-15 tires would be turning 1821 RPM in fifth gear and 2428 in fourth gear at 65 MPH, so your proposed setup is actually a slight improvement over a stock five-speed. Yes

Doesn't have to be '87 - '90. The older ones ('84 - '86) will also work if you change the speedometer cable. With the older ones, you MUST get one from a vehicle with the same number of cylinders. The older ones had no calibration potentiometer. The newer style have a potentiometer that allows calibrating for 4-cylinder or 6-cylinder.

What year, and do you have a functioning axle disconnect?

Why not?

There's nothing really wrong with that. It allows the use of the stock shocks.

Not great. Rosin-core solder is used for electronics soldering and not generally used for other types of work. If it doesn't say "Rosin-Core" on the label, it isn't rosin-core. Radio Shack is still the source for electronics soldering supplies IMHO (until they fold completely, which could be any day now).

I'll add another vote for air bag spring helpers. Not a factory option. As has been said, photos would help.

Without replacing the springs, the absolute minimum lift from a SOA conversion is 5" to 5-1/2". If your springs are sagged, then doing a SOA plus new springs will definitely generate more than 5-1/2" of lift.

It all depends on how you intend to use the vehicle. Let's face it, there are tens of thousands of XJs and MJs in the world with the axle disconnect, and they work just fine. My '88 XJ has it -- 287,000 miles, has been wheeled at Paragon, on the power line rail in northern Rhode Island, Old Florida Road in Massachusetts, through one of the state parks near the Connecticut/Massachusetts state line, in Maine, and on fire roads around northern New Mexico. Still has the disconnect. The '88 MJ has fewer miles, but I bought it used. The previous owner wasd a teenager who had beat on it hard enough to blow up the Peugeot transmission -- but the disconnect axle was (and is) fine. I've wheeled that at Paragon, Old Florida Road, the Rhode Island power line trail, and some local spots nobody has ever heard of. Is the one piece axle shaft of the late model XJs stronger? Maybe a little, but remember -- it's the same diameter. You can get late models axles with larger u-joints, but just the absence of the disconnect isn't a guarantee of that. Lastly, the passenger side control arm mount is significantly WEAKER than the one that's part of the disconnect housing. What are your intentions for the vehicle?

Sometimes the only answer necessary is "Because I can." If you plan to do a good job of it and not a hack, I'd say go for it. Just be aware that cooling has always been a problem even with small-block Chevies in XJs and MJs, so pay attention to cooling.

I wonder if Hell Creek has different source information than General Spring. When I kvetched about the fact that most people find that General Spring's metric ton springs result in about a 3-inch lift, General Spring chimed in with some information purporting to show that there were actually a number of different metric ton springs from the factory. This would almost have to have been due to different option packages, because the factory service manual doesn't mention any difference in ride height for the metric ton package. They list a ride height for 2WD and a different ride height for 4WD. Period. (Both are subject to a plus-or-minus 1/2 inch tolerance.) I typically defer to the FSM. If they don't provide a different ride height for the metric ton package in the section specifically devoted to checking ride height -- then that suggests to me that the metric ton springs were intended to ride at "stock" height while providing more capacity.

The stiff pedal makes it sound to me like the straps on the slave cylinder didn't break.

The 90 4.0L didn't have a temperature sensor in the thermostat housing, the one for the gauges is in the left rear corner of the head and the one for the ECU is on the lower left side of the block. It shoulds like someone just used a 91+ thermostat housing on an older vehicle.

^^^ This. Do not drill the hole in the perch on the axle. Use a large C-clamp to hold the leaves in place and compressed while you remove the center bolt/pin, grind it, and replace it.

The only Cherokees with a foot brake are the police models with bench seats.

The solenoid is built onto the starter.

If the brake warning light didn't come on, I don't think the shuttle valve in the front distribution block opened the bypass line.

Beats me. The OP is the one who wants to raise his shock mounts. Mine never dragged when I wheeled the '88, but maybe I'm a wimpy wheeler.

Find where the drain tube attaches to the air box and see if the tube has rotted or split.

That setup is great for street use, but it doesn't improve the ground clearance at all compared to the stock mounts. Might even be worse for ground clearance.

Consider that you may have a bad ground at the back, and the backup lights are getting power but no ground. Do the taillights, brake lights, and turn signals function correctly?

You put the photo(s) on a photo hosting site like Photobucket or Tinypics, then post a link to the photo(s).