I'm puzzled

[kaptain Jack]

Picked up a bargain, a high lift.The guy admitted that shifter is finicky. Brought it home, the problem is the higher gear doesn't engage, high pitch sound and nothing, lower and second ok. Took the gear box apart, seems to be ok, added grease, tested manually,works fine. Rear axle was a bit stripped, but not much, changed the parts. Same problem: upside down, all is fine shifting and engaging ok, on the floor only low and second, high slips with high pitch motor.
Any ideas?