This is just my guess, and may sound insanely simple (because the entire nature of your post is something that I have no idea how to do), but do you have full time 4X4 on that thing? I only ask, and I could be totally off on this, but I don't think that U-joint axles enjoy being at a full right or left lock on dry pavement, or anywhere where they can't have slippage...
It may well not be that at all, the only reason I bring that up, is because the symptoms that you describe are that of me putting Jeepy into 4X4 (I have the part time version), and trying to drive at full lock on the dry pavement, maybe CV shafts are capable of this, while U-joint shafts are not... This could also very well be the differential, or even the
transfer case that makes the difference between the GCs full time 4X4, and the XJ's part time 4X4, in which case, I would be totally wrong, but if anything, it's something to mull over, and I'll probably learn a bit of something too :)