I think you're going to be disappointed with all that unsprung weight. What tires are you planning on running?

That being said, I don't think the u-joint yoke is going to be your biggest problem. You can always have a new yoke welded onto your driveshaft that will match the yoke on the axle. You may have to shorten the driveshaft anyway, so that could be done at the same time.

I'm surprised that the spring pads on the D70 axle line up correctly with the springs on your XJ. Assuming they do, how does the overall width of the axle compare to the width of the OEM axle? I'm assuming that the D70 is a full floating axle, as most one-ton axle are. If you have to shorten the axle, then shortening the axle housing will be the least concern, cost-wise. Getting shorter axle shafts in the housing will be a higher expense.

Also, don't forget that the bolt pattern of the new axle will undoubtedly be different than the bolt pattern of the OEM axle. That means you'll have to buy new wheels (and probably tires) that will fit the new axle's bolt pattern (and get them with the correct ofset, which may involve getting expensive custom wheels). And, after doing that, you'll have different wheels on he rear of the vehicle compared to the front, which means that ou'll have to elect which type of spare tire to carry, because the spare will only fit one of the axles. That consideration alone would make me look at the possibiloity of changing out the front axle, too.