My friend has an '88 Toyota p/u, 22R-E, manual transmission, 4" suspension lift, 3" body lift. The truck has a tendency to pop out of gear on its own, especially first, when under load. If you hold the shift lever in place, it will stay in gear, but once you let go and get on the gas a little bit, it will pop out into neutral. It shifts into gear just fine, it seems. There is no shift boot or anything that might touch the shift lever when the driveline moves. A search on 'transmission popping out of gear' didn't yield any good results. Anyone have any thoughts on what the problem might be?

There's a wear pattern that can develop on the leading edges of the synchronizers that will cause it to pop out of gear..

I just had the same problem fixed with mine. It started in first, then moved on to third, and got really bad.

It was the bearings in the transmission on the shafts that the gears are on. It was allowing them to slide with the clutch was let out, knocking it out of gear in gears at the top of the pattern.

One way to check would be to have him drive and shift into second and hold the shifter. When he lets the clutch out see if the shifter moves forward in his hand.

It wasn't too expensive to fix. We took it to a mechanic who knows toyotas and he said it was common. He just had the truck one day.