have inherited a family owned (and well maintained) 93 trooper

based on meticulous records, engine oil, filters, plugs, harness, axles,
boots, differentials (dual since 4wd) and finally the transmission, have all
received a regular inspection or parts or fluids change

car runs and sounds like new but am a little concerned over the transmission
as it does sometimes lock into 3rd gear which is a known GM transmission
feature that is designed to protect the transmission if something is not
right.

the fluid is clear, no odor, pan dropped and filter replaced couple months
ago, replaced about 9 quarts of the total fluid capacity at that point.
there was hardly anything inside the bottom of the pan, a little amount of
residue at the bottom which I think to be some wear from the surfaces and
carried by the fluid to the bottom of the pan.

transmission shifts smooth, makes no sounds and drives for a month without
incident, then all of a sudden when coming to a complete stop at a red
light, the red transmission indicator (check trans) comes on and the 3rd
gear lock is engaged. it is reset by simply turning the car off, restarting
engine and all is well again for another month.

if I replaced it with a rebuilt transmission it would cost about 1200-1500,
parts shipped to me and I would have to ship my existing tranny back within
30 days. if I took this to a local transmission shop, I can foresee them
very quickly spending that same amount in diagnostics and not sure what else
to consider.