Don't take this the wrong way, but have you actually FULLY considered what you're going to do with your XJ? I concur that more than 3"/31's on a DD is overkill, and is going to cost you more in the long run. Besides, my 87 ran 3"/31's/D30/D35 (3.55) and was really quite capable offroad, and well-mannered onroad. I did consider going to 4.11 or so for axle gears, but I never got around to it (and now it's a project truck, and will probably be unrecognisable when I finally finish it in a few years and somewhere outside California...)
I'd not consider a lift something to do lightly, and going with bigger tyres will just murder your fuel mileage (especially with an automatic - the average auto eats up something like 15-25HP just spinning, vice the 2-3HP for a manual. That's why manuals get better mileage, all else being equal.) I have nothing against the AW4 four-speed auto - I really think it's one of the better slushboxes out there - but it's something else to think about. If you are going to do all the tyres and lift you are thinking of, consider going to an AX15/NV3550/NV4500 manual as well. If you don't know how to drive a manual, you really should go learn, as it really is something every serious driver should know!
Moving on - I've got 5.25" Sony Xplods up front (OEM size) and 6x9" Sony Xplods out back (some mods required to liftgate) and they tuck quite nicely behind the OEM grilles and trim panels! I'd check out the charts at crutchfield.com, they will tell you OEMR size (which doesn't matter) and mounting depth available (which damn sure does!) so you can decide how to mount what you've got and if it will work well. There are also some interesting options for driver mounting available for you, IIRC - I think you'd start with QLogic for those. If you have to rewire, and especiall if you have to drop an amp for a driver or two, check both doorsills, and run the audio leads down the side that doesn't have the main harness (usually the driver's side.) Don't be stingy with the wiring, either.
For drivers, I highly suggest leaving yourself the ability to pull the box when needed - you might find that to happen quite often. I found a nifty little "double banana" jack plate at Radio Shack for a few bucks, and they have twist-locking banana plugs as well. Both are gold flash-plated, and offer good connexions (I use a couple for auxiliary power as well, when I need something REALLY secure and a lighter plug just won't do.)
Finally, remember that most of us here are oddballs, and take what we say with the requisite grain of salt. Put on your thick skin and continue to ask questions - you will get some useful answers if you take the time to ask. Also, take the time to THINK before you WORK - you might save yourself some trouble and some knuckle agony, and you just might find out a new way to do something and make your life easier....
5-90