Drive wheel seize at low speed

I have a 1994 Silverado 1500 pickup, ext cab short bed 4x4 with 116k. The motor is a 350. The rear diff is open (pretty sure its the 3.71), rear drum brakes. I bought the truck used just a few months ago.

The problem is that at very low speed (<5mph)during turns, the rear right (drive) wheel will lock up sometimes. And its not a slow lock up, but a HARD clunk lockup. The entire drivetrain appears to lockup, and you must releive the pressure on the drivetrain to get it to go again (either come off the gas, or shifting into neutral or reverse sometimes). Also, this problem is not present all the time, sometimes the truck will be just fine for ~500 miles, other times it will do this 3 times in 20 minutes.

One of my buddies suggested inspecting the drum brakes in the back, thinking something would be getting jammed in between the drum and shoe, but I opened up both on the back and they look clean (no wearing on the drum or shoe). It also appears that the wheel bearings are in good shape.

I'm completely out of ideas on what to do here, any help you can give me would be greatly appreciated!