I agree with Frank50.

Jim, can you post another photo that shows us more of the drive shaft AND the rear/attachment at the transmission?

A long wheel base 1/2 ton GM of that era had a short, enclosed extension that I think had a u-joint in it.

I will see if I can find a photo on oldGMCtrucks discussion board. You should consider joining that group.
Attachments
GMC longbed u-joint parts diagram.jpg (39.34 KB, 76 downloads)
GMC longbed u-joint parts 2.jpg (27.26 KB, 77 downloads)
GMC longbed parts 3.jpg (23.98 KB, 77 downloads)
GMC longbed u-joint parts 4.jpg (53.45 KB, 80 downloads)

Last edited by tclederman; 09/20/2019 4:52 PM. Reason: photos added