The oil is probably too thin, it should be changed every 2 years/24,ooo mikes but as these cars get older people havn't always bothered.
Use ATF, Dextron2/3 (I use Lucas racing ATF) and as with Auto boxes a tin of "yak" might also help with shift quality.
Check if your car was built with an oil cooler, some were, for the gear box and if so that the pipes aren't damaged or blocked in any way. It won't use the water rad but will have the hairy sausage ahead of the rad.
At the end of the day the LT77 is a Sherpa van box and if you get 80,000 miles out of it you are doing well.