Thrust cap damage

[Wolfman3002]

Hi Y'all.
I'm in the process of changing the rear end on my '73 GT for one out of a donor car (also a '73). I've had a constant whine from the differential since I bought the car (a little over 2 years ago), one that increases and decreases w/ the speed of the car, and doesn't change when I corner left or right. I have since rebuilt the central joint, and it made no difference in the sound. So, I figured it's the pinion bearings or maybe worn gears.
Anyway, I removed the donor rear end, took off it's torque tube and central joint, then cleaned and painted it (they look so nice when they're clean and painted, don't they?). Then, last weekend, I removed the rear end off my driver, removed the torque tube and central joint to move it over onto the donor rear end. Then, I looked at the thrust cap on the end of the pinion shaft sticking out of the differential, and it looked like someone had taken a ball peen hammer to it, all over, and it also has a score mark all around the circumference. I've attached pics below of the damaged thrust cap next to the undamaged donor one.
So, I'm trying to figure out if the bearing or gear problem would cause this damage, or if there is something else I should look into while I've got it apart. And, before anyone asks, I do have a thrust spring in the tranny output shaft. One other thing, in case it may be a clue to someone. After I unbolted the universal joint from the central joint, and tried to push it forward against the tranny thrust spring to disconnect it, it acted like it was stuck and the spring would not compress. After wiggling it around a little, it came loose, and I was able to push it forward like normal, and then it moved in and out of the output shaft w/o resistance (except the spring resistance). Is this just a result of the bad gears in the differential, or a clue that something else may be wrong?
If I can't figure anything else out, I'll just continue w/ the swap, and see what happens. I just don't want to ruin any good parts because of something I don't check out.
Thanks,
Randy

[SpringGT]

That sorta just looks like pitting corrosion on the thrust cap to me. The cap just fits into the barrel spline of the torque tube drive shaft and there is no rotation of it relative to the shaft, so it shouldn't be a case of brinneling or spalling.

However, if as you say the front driveshaft spline was frozen to the transmission output spline, maybe all of the sliding motion in the driveline normally accomodated at the transmission tailshaft was transferred back to the back and the thrust cap was hammered against the inside of the barrel spline on the torque tube drive shaft. Odd, huh?

[Wolfman3002]

Yah, and one wierd thing, is that inside the end of the pinion extension shaft is a rubber seat and then a metal disc behind that, as a kind of bearing surface I guess, and while the metal disc was also peppered with dings, the rubber seat that sits between them was not damaged much (although it did look a lot thinner than the one from the donor car).

[markandson]

That does not look like "hammering" to me, it looks like corrosion/rust/galvanic action of some sort. JMO

[wrench459]

Or could it be..
Lets put in todays terms ..left inner c/v tri-pod wears out causing the right side to move the same amount..where is the bad part?
same with the diff. be it a worn front pinion bgr. or rear bgr. its still the same story. The damage has been done.
Time for a rebuild.

[Wolfman3002]

Good thing I don't have to do a rebuild, since I have the donor axle (which I think/hope/pray is good). Although, if I ever get the time, maybe I'll do a rebuild, or at least a tear-down, of the old axle, to see if I can find what the problem was. It was just much cheaper to buy a good donor car, with all it's extra parts ($160!), than to buy all the expensive replacement parts needed for a rebuild (again, assuming the donor car's rear end is good).
Randy