You need a new wheel bearing on the back,jack each side up and spin the rear wheels and one will make a rumble noise thats the one that needs replacing
gears slipping out will be indicated by the gear lever jumping into neutral.This can be from incorrect rear engagement from a loose worn pivot in the gear lever tower or something stopping the gear lever from moving properly.. Clutch problems are indicated by --going to the floor. crashing 1st gear, engines rpm s climbing without an appreciable vehicle speed increase. If there is a fault in the gear box it will be only in the gear engaged (missing teeth-chipped ) and not all gears. If it is a bearing then there will be no noise in 4th gear
Boxes are repairable as mostly it only needs that faulty part to be replaced