92 firebird 5.0 tbi manual trans jumps 5th gear to neutral

Feeling vibration when driving . Slight
vibration  in nuetral as if a misfire. I hear a
clunk when down shifting in the rear of the
trans.  Can feel movement in the rear end
of the car on some occasions. Exhaust is
vibrating on and off. Replaced the rear tires
and trans mount and still jumps out of 5th
gear.  Sometimes I feel it trying to jump as I
hold the shifter more than other times.
Shifts easily and no grinding . I noticed a
wet slave cylinder and lots of play. This car
was parked for a long time in a auto pound
that the clear coat faded. The drive shaft
looks rusty and so do the unjoints. When I
bought the car there were two new motor
mounts and the trans mount was broken.
I'm hoping that replacing ujoints, motor
mounts and slave cylinder would solve this
problem along with a new set of plugs,
wires and cap.... when on 5th gear it runs
really rough and vibrates bad. Has anyone
had this problem. Advice needed . Thank
you