U-joints failing

Hello to all
First of all I would like to thank all of the people that have provided information to this board, it has been a wealth of knowledge to keep my XJ running and the wife off my back for having another broke vehicle in the drive.
I searched but haven’t found what I am experiencing from my 88 XJ, AW4 4X4, with stock suspension and 235/75R15's, no lifts no nothing in change to the driveline.
My problem is that the XJ eats the U-joint in the back drive shaft near the Diff, the front one is fine by the trans no problems ever. I have replaced them together just to eliminate the thought of the front being bad and possibly having some effect on the rear one. Normal simptoms when it goes out vibration increases as speed does, it sort of smooths out at 65, but when i get under the jeep i can see the joint flex, whan i swist it. Or in the worst case not a lot of vibration but a lot of physical damage to the caps can be seen, just by looking you can see metal flaking away from the edge of the caps. New U-joints each time, but the same result is the back one goes out in less than a year. I don’t wheel it; it’s just my DD at the moment.
I have checked for signs of damage to the driveshaft and yokes, nick, dings or dents but find nothing to that effect, I can’t see any place where a weight might have slung off. The yoke fits tight to the caps of the U-joint every time I have installed them, and they get a little grease just about every time I’m doing maintenance now " read that as chasing oil leaks " just to make sure they haven’t gone dry.
I even make sure the straps everyone seems to hate are tourqued and in place so the U-joint doesn’t take a lashing as torque is applied.
Any ideas on something I’m missing? It’s it possible to get it in backwards and have this problem? Or should I just take the overkill route and get a new driveshaft?
This one is the only rear wheel drive I’ve ever owned that has done this to me.
Any personal insights or experiences would be great as this is to simple to be as hard as it seems to be at the moment.

First move: take the drive shaft to a drive shaft repair shop, and have them rebalance it- shouldn't cost much. Describe the problem to them, they may know straight off. If that doesn't do it, then I would suspect slop in the rear pinion bearing....but you should be able to feel that by checking for lateral (not forward-back) movement by grabbing the yoke with the driveshaft off. Something is seriously out of spec: my 1990 with 230,00 km (144,000 miles) is only on its second rear driveshaft u-joint.

X2 on the pinion bearing going out, after you drive awhile, like say, Kelly AFB to Red McCombs Ford, crawl under and feel the pinion snout. It should be a little warm, not hot to the touch. Ren

Thanks Bob and Ren, I just got to it tonight and checked the pinion when i got the driveshaft out, it didn't move from side to side at all no matter what type of pressure i put on it. It wasn't even really hot after the drive home which is about the same distance Ren sugested.
I don't think it will matter at the momment as this time it took a chunk out of the rear yoke and I'm going to need to replace it or the driveshaft, I'm betting a junkyard special will do, there is enough XJ's in the local yards.

Thanks for the sugestions, but I'm betting the driveshaft was at fault here and i just couldn't see a crack or something, not enough force to spread it or something from just me twisting on it.

Thanks Andy

P.S. Ren it say your in Ky, when were you running around the 410 loop?

Well, the last time that I was HOME, China Grove was still outside the limits of S.A. I am orginally from over by Charlotte, if you are leaving Poteet or Jordanton. I have kin down in Pleasanton, on top of the hill, coming from out of town and over the tracks and the bridge. If you ever get up this way-Ft.Knox, or what ever, let me know and I can show you the Red River Gorge and Ky. Offroad Park. They are just around the corner and are great places to 'wheel. Ren