My 92 2.8TDX (leaf sprung) Fourtrak/Rocky has developed a grinding / beating habit in 5th gear. Transfer box is an unreconditioned breaker box after an accident last year. Its always been a bit rough but I was wondering if it is the main shaft bearing that is on the way out this time. The vibration kicks in if I go over a bump in 5th and I have to back off the power and rev up again to keep it smooth. Also if I engine brake in 5th I get the same. I guess its either 5th on the way out or the main shaft. the whole transmission is a bit iffy above 2700 revs anyway.
Any ideas before I start a major removal and strip down job?

The 2nd problem is my girlfriends 91 petrol EFI sportrak. Its got a severe flat spot between 2000 and 4000 revs. Feels a bit like the misfires my old citroen had when the coil pack was on the way out. Anyone had this with a sporty or know of a/what to fix.

Thanks people