gearbox issue

[92 triton 2.5Td]

Hi fellas, new to this site and getting some really good info but not exactly what i need to know. would be good to hear if someone can help! I have 2 tritons and my girlfriend has one as well. she has a 95 mod 4x4 2.6ltr double cab with a strange problem with going into gears. We almost made it to the goldy the other week from 600k south and in-between 1st and 2nd gear changes the gearbox jammed. this stalled the car and skidded the car to a stop . the gearstick was locked but later tried everything to move it and forced the gearstick to then free the gears and resume on our way. in the months prior to this it was becoming more and more difficult to change gears as if all synchros were stuffed or possibly clutch not fully engaging or something. clutch is worn and will be replaced but this shouldn’t cause the problem?? it changes gears easily if you match revs to the correct rev to match the ground speed for that gear. grinds alot and hard to get into reverse. I have replaced the nylon bush and end plastic cap thing on gear stick itself thinking it could help but still has same issue. need to know if my spare 92 2.5td gearbox will fit it also is it may be a solution?? i can fit it my self by the looks of it. all ill need to do is swap the cable activation to hydraulic to match the bell of the diesel gearbox. also thinking of swapping internals of box to but prob a big job?? and may not solve my prob ? possibly the wrong gear ratio's ?

thank's aaron.