Engine transmission lock up

My initial thought was an engine issue as well, like a broken rod, or interference with a failed valve. When reverse rotation cleared the lockup, and the engine ran fine with no issues, my thoughts started drifting away, from being an engine issue. About 3 weeks running, with normal performance after the flex-plate had been replaced, I once again experienced the lockup. I removed the serpentine belt, to eliminate the alternator, water pump etc being. The culprit, and the lockup again was cleared, by rotating the crank in reverse. Again the van started and runs, with no apparent engine issues. Granted I have thought both about the oil pump and timing chain, but because it had been put in neutral, and allowed to roll a couple feet and then put in park prior to both lockup situations, my suspicions were directed to it being a torque converter, transmission issue. I am open to any theories! Replacing the flex-plate again is haunting!

Thank you for the reply, and explaining the transmission workings. Having built race engines and manual transmissions for my son, as well as several other engine/manual trans rebuilds over my 50 year mechanic history, I rate my ability higher than a normal weekend DIY. With that said. I have very limited auto transmission experience. With the focus now back to being an engine issue, I would think a crank or rod bearing issue, would be detectable under operation. Although the alternator pulley turned easily clock wise when the belt tension was released, I am now wondering about the alt bearing locking when under tension. I could hear a distinct "clink" during the first lockup, when going back and forth rotating the crank, at the forward lockup point. Other clues may be, that the 3.8 has an exterior oil pump, being driven from a 3/8" shaft coming for the engine. I sort of eliminated the pump, thinking the shaft would likely fail, if something was in the pump gears. Virtually impossible to do, but I feel like I need to manually rotate the crank, prior to engaging the starter.

Will do! It would seem that something has to move or settle in while just sitting there, as the engine rotates freely while running, prior to being stopped and the restart attempt. I hope I do not need to, but if I experience lockup again. I will be checking numerous items before any motor rotation is attempted!