What about drilling straight through the frame and running bolts all of the way through and bolting them on the inside with fender washers or reinforcing plates? With or without sleeving the hole ( I hate nutzerts :) )
$3700 with a bad motor, t-case problems , and busted window? This guy is trying to flip a truck he bought cheap IMHO. If it is the truck in the ad it is not worth anything near $3700 of my dollars.
For those of you too young to remember the XJ was universally hated by jeepers when it was introduced. Worse than the YJ even. It wasn't until second and third owners began beating the crap out of them that they felt the love.
Have you considered an 8.25 w/29 splines or 8.8? Cheaper, stronger, & easier to find. The MJ/XJ D44 is weaker than the average D44 ( smaller tubes) and is overpriced IMHO
As mentioned it helps because it moves stuff that will get hung up on things above the axle. It also allows you to use flatter leafs then what would be needed from a SUA to get the same lift. High arches take away from flex.