The reason 1.25" spacers are so popular for the stock Jeep rim owners is because the factory rim has 5.25" back spacing, so with spacers, you'd go down to 4.00" of back spacing. At least that's always been my mentality. Not sure why you want to have the rear rims spaced but not the front (unless I read wrong).
As for the gears, 4.10's would be ideal with 31's on a 4.0L / AW4 equipped Jeep, but not inherently necessary. I ran 3.55's on my XJ until I so happened to stubble upon a 4-banger / 5-speed XJ at the junkyard, on a 50% sale no less, so I couldn't pass up the opportunity to pull them and swap it in my rig. I now run 33's and I hardly notice a difference in pulling power.