Usually if it is jumping out of the 2 back gears 2nd and 4th. That means something isn't lined up quite right, and the shifter is hitting the body. Most common with a YJ that has a body lift on it.
You can try taking the floor pan out of it, and take it for a ride. If it cured the problem, then you just need to trim the pan to keep it from hitting. If it didn't cure the problem, then I would say the synchronizers are going in those gears although 4th is an extremely odd gear for the synchronizers to go bad. Most common are 2nd and 3rd.
There is no adjustment on a hydraulic clutch. Other than bleeding the system to get all of the air out. If you can start the vehicle and then shift into 1st gear without any grinding, that is a sign that the clutch is bled out properly.
'87 YJ, 4.2L built and bored, 8" of lift, 8.8 rear, AX15/NP 231J, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...