From what I've read, the drive shafts for a Chrysler 8.25 diff are shorter than those for a D35. That said, I don't know what the right length is - apparently there are a whole lot of different transmission / transfer case / differential combinations, and therefore a lot of different driveshaft lengths that were supplied. I guess the key is to get a feel for whether the splines are nearly or fully engaged with the shaft in place (before the tranny is put in gear). On mine, I can remember being able to move the shaft forward quite a bit more than necessary to fit it into the rear U-joint mounts (I'm thinking by 3" or so, maybe more than that). I guess if you'd noticed that you didn't have much play when you dropped the shaft (that is, you could just barely get the rear U-joint into the pinion carrier), then it's likely your rear driveshaft is too long for the application. Hopefully someone here has the same engine / tranny / transfer case / diff as your XJ and can do a quick measurement to see if your shaft is the right one (or not).
I'm having trouble thinking of what else it could be - if the motor and tranny mounts are in the right place (and I suspect they are!), and we KNOW they're good - AND the radiator mount and shroud are in the right place (and it sure sounds like it), then all that's left is something that's "pushing" that tranny/engine assembly forward. I can't think of anything in the engine / tranny / transfer case that COULD push the engine forward, at least not with good motor and tranny mounts. That leaves only what's behind the transfer case, and that shouldn't be possible unless the "push" is in excess of what is taken up by the splines in the driveshaft coupling, which should give you at LEAST a few inches of slop before anything gets pushed around.