I recently rebuilt my M5OD-R1 from my 95 Ranger, 2wd 2.3L. I replaced all of the bearings and seals, but not the gears, shafts, or synchros. Everything seemed to be fine before I put the trans in, but now it feels like there is a brick wall blocking 4th gear. It shifts into every other gear smoothly and drives great, no strange noises or anything.

​​​​​​I should also mention that all of the gears shifted perfectly right before I re-installed the transmission (trans sitting on the floor with shift lever temporarily installed).