Throw some ideas out...

Passenger-side rear brake drum mysteriously developed an issue while truck sat up during recent motor swap. Seems like it's stuck in "apply", thus locking only the passenger-side rear wheel. The other three wheels/brakes are fine. What's wierd is the fact that it only does it in DRIVE. Seems "free" in reverse. This is a non-ABS application. For those that don't know, the rear brakes are drum brakes (unlike newer SUVs).

My suspicion is that drum's cylinder is bad (each drum has a hyraulic cylinder). I'll try to take a pic this evening. I replaced the driver-side cylinder years ago b/c it leaked severly. I'm thinking the passenger-side may require changing as well.