It really does seem that your whole transmission is so worn that it would be uneconomical to repair.It has been too many years since I had my oily mits inside an LT85 but recall that a circlip on the mainshaft that holds 5th gear into position can fail and allow the shaft and gears inside to float back and forth on overun, with the possibilty of partially engaging 2 gears
simultaneously, which would lock up the transmission and stall the engine if it didn't break a gear first.
Wagoo.

