Technical 51 Plymouth lowering question

I'm more surprised nobody commented on this. Lots of stock shafts are out of balance.
I might talk to a different drive shaft shop. If you go in and ask them to check the balance, they should chuck it up and do so. It doesn't take long and doesn't cost much just to check it.

But if a guy told you it didn't need checking just because he didn't see any obvious bends, welds, etc. is just silly. He has no idea what some former owner had done to it. If a good welder did it, and it was painted well, you wouldn't even notice his work after a good coat of road grime.