S.O.S. 833 pops out of 3rd without load.

[Jim B]

Is this a trans repair on an existing past functioning trans/engine combo? I'm asking because if it's a conversion you need to check the crankshaft for a pilot bearing. (bushing)
OE production line automatic mopar cranks may not be machined to accept a pilot bushing and putting a 4 speed behind it without a bushing leaves the input shaft without front support. That usually results in a vibration but I'm thinking maybe on a fresh rebuild the "wobble" is covered up for a while until the front bearing/input rollers wear a bit. You have checked all the synchro assembly and side cover detent areas that I would have been sure was the problem area. Since back in the day the 3-4 synchro hub/sleeve/dogs/springs was serviced as an assembly I never had to check for mismatched components. I don't remember if it was possible but I assume someone verified the hub is not installed backwards or has excessive end play and the correct snap ring is installed. I assume you have good detent action (sleeve/balls/spring) in the side cover when off the trans.

I remember the 18 spline trans having a coarser pitch on the gear teeth than the 23 spline unit but I don't think the wrong gear will mesh with the cluster gear.
You aren't going find a more experienced person than Arlen on these units so unless I am missing something I'd have to compare the new parts with the originals to try to find a bogus piece.