if you are unsure when the drivetrain oils were last done I would "baseline" the lot.
diffs, gearbox, transfere box. every 40000 miles. diffs, front in particular, more often if your 80 likes water.
I change the engine oil and grease the UJs every 5000 miles. genuine fuel filter every other oil change.
timing belt every 60000 miles. I change the auxiliary belts every other timing belt change. not had one fail yet.
again, if your unsure when the coolant was last changed give this a flush and fresh coolant.
check the wheel bearings for play.