You can probably make a D44 with chromo's and CTM's last with 39.5's in the mud, but not in the rocks. 37's are it for a well built D44. Personally, I've broken my front D44 more since I went to 37's than I broke my D30 through many sets of 32's and 33's. The new D44 CTM's are very stout, but even chromo yokes have a breaking point, and what breaks most is the stub shaft and hub. You could go with CJ spindles and do a Warn 30 spline kit to get chromo 30 spline stubs, but that's even more money. Me, FarmerMatt, Paul S., Dave T., Cracker, and others have all run 37's on a front D44, and we're all going front D60. I wish I'd never put the money into the D44, should have gone D60 from the beginning, but I had this illusion that I would stick with 35's........yeah, right!
If 39.5's is what you want to run, you have no choice but to do a D60, or some kind of D60 hybrid. If you have to go cheap, then you're only choice is a Ford D60. Shop around here and on Pirate and on E-bay and buy one, it'll cost you around a grand. Run it full width if you want, get some Spicer 35 spline stubs from Part's Mike or someone, get a 14 bolt for the rear and weld it, get some 8 lug rims, and give it hell.
You can put a four link on anything, hardly anyone building any more uses leaf springs. You'll need to reinforce the chassis of the XJ, not because you'll have heavy axles underneath it (don't know where people get that idea, the rig sets on the axles, not the other way around) but because you're going to wheel it hard and it will fatigue crack like crazy after awhile if you don't stiffen it up.
it will be fine.
