I did change a lot of rubbery things, fluids, etc, added some modern chemicals... brakes etc, you dont tel which transmission you use, mine is the 3sp TH400, I had once an issue where suddenly the trans was stuck in first!, (short story...) the governor gear (a punny little plastic gear, $2 in ebay) striped... very easy fix even on the road!, its a cover held by 4 1/2" bolts on mine is on the passenger side of the trans, also change the vacuum modulator, on mine was ok but Ive heard about diaphragm rupturing and then the engine sucks the ATF leaving the trans dry and NOT MOVING! (yours I think is electronic, so only change the governor gear in a 4L80 is on the DRIVERS side)
Ill do it anyway... put a new (reman) starter, and
NEW alternator!!!!
put
MOTUL oil in your differential.
all your efforts should be focused first on your safety an secondly on the longevity trouble free of any component
HOPE I HAVE PUT YOU TO THINK!!!
GREETINGS FROM SUNNY GUATEMALA