Gearbox problems and solutions
The LT77 (Reverse by 1st gear) gearboxes had a design flaw in the output shaft that goes to the transfer case, then the transfer case input gear had the same flaw, the thing is, that these parts are not properly lubricated and the splines wear faster than normal. The R380 (Reverse under 5th gear) gearboxes had a revised desing with drilled input gear and some different seals to avoid this problem.
An easy way to identify the problem is you will hear a clunk in the transmission when accelerating or under hard breaking.
*Don't confuse with the rear suspension arm ball joint clunk, the ball joint clunk will come from the rear.*
Solutions:
I had to replace the transmission output shaft and t-case input gear with the revised parts in my Defender 90 (parts cost was about U$500), the splines were really worn so this was my only option.
If you have an LT77 in your vehicle and you don't have the clunk yet, you can buy a kit which will replace some seals and gaskets, these will divert oil to the problem area reducing wear of the splines.