I just sold the 500 Caddy engine & TH400 I had in my shed 10 years ,back to the guy who I got it from--I had the same driveshaft too,the drivetrain was from a 76 Coupe De Vlle--I always wondered why no one had tried using a shaft like these in a lifted 4x4 ...as for them not being "rebuildable" I think the only thing that would prevent you from rebuilding one,is if the stub that rides in the ball can wear down,to restore it would require spray welding it back up oversize and then turn it down on a lathe to the right diameter--perhaps a bushing could be used instead?...
One thing about driveshafts--ever since junkyards started using forkloaders,its dam near impossible to find one thats not kinked in half any more!...usually you end up buying a new one for a kings ramsom,or taking the muckled remains of the junkyard one to a driveline shop to be re-tubed..the last junkyard around here that removed driveshafts before lifting the vehicle off a ramp truck with the loader and saved them is now clearing out all their "old" stuff,including the stacks of driveshafts piled up like cord wood behind the office trailer...I lucked out and found one for my '81 G-10 van there many years ago,when I discovered mine was dented..good luck finding one any good around here any more since though--you practically have to buy a whole truck just to get one,before it gets near the junkyard if you cant afford a new or retubed one!..