driveshaft------shafted?

[Former member Yoder213]

New here after some months of watching from the sidelines
I got a 1970 C20 FOUR WHEEL DRIVE (that's what the badges say, doesn't sound like anything I've seen posted about yet)
He's got the 292, bored .40 or .60 over
4 on the floor, 4.56 with factory Detroit locker and impossible to find drums.

I'm the third owner, PO had it since '73, added power steering, switched to HEI, and added a couple gas tanks. I love the truck and plan on caring for it like he had, but...

I've developed a strange, or what I think is strange, sound and behavior in my driveshaft.

when parked, (cant remember if it was in gear or not) I can rotate freely my driveshaft what seems to be a normal distance counter clockwise (looking at the pumpkin out through the back of the truck). then it tightens up, makes a loud "tink" that sounds like it comes from the shaft, then it frees up and rotates another distance equal to the first.

Where should I start looking to figure this out? I can hear the noise happening when driving and at times where I think the shaft would be allowed to "play" but I wasn't raised around a mechanic and try to absorb everything I can from the older mechanics and ones who sound logical, like these forums hahahaha

thank you all, I plan on becoming familiar with this site as I restore my truck.