1981 GMC K20 4wd Driveline Problem/Issue?

[Silvy06]

First I should start by saying I'm not to sure if this is in the right thread because I'm fairly new to this forum.

My question is I just recently bought a 81 GMC K20 and I'm new to the Solid Axle truck game. So far everything seems reasonably good except the U-joints in the Steering Knuckles and I might have to find a new yoke for the front axle. Was wondering about any pointers on what else to look for in these trucks axle wise I don't know what is has under it for axles yet haven't looked for tags.

Another question is that when I put the truck into gear from Park to Reverse or from Reverse to Park and or back it makes a clunk and a slight jolt. I've searched on here and some said high idle and thats not the issue seeing as its pretty low. The U-joints for the rear axle seem pretty solid no up and down or side to side movement. But I had the gf switch it from drive to reverse for me and the rear axle makes a 1/4 to 1/2 turn when it shifts into gear.

I should also mention that its a TH350 Transmission. I'm not to familiar with em so I'm not sure if thats how they shift into gear. I know the previous owner said he replaced the modulator.

If anyone would like to point me into a good direction on what to look at its greatly appreciated.