check the transmission fluid.... if the fluid is gone, its time to replace the transmission.... if you have more than 150k on it then you should replace it anyway, as automatic transmissions, especially american ones, dont typically fare well after long use. Are you sure its a 4-spd? if you have an overdrive than i think it is 4-spd, i dont believe the 3-spd had an overdrive, but im not sure.

 

I had the same problem on my 95 GC. Took it to a transmission shop and had it checked out on the scanner. Turned out to be a turbine sensor. The shop charged 75 for the diagnosis and repair. Figured it was worth it.