Transmission slippage

1994 Jeep Grand Cherokee Limited, V8, around 200k miles now.

Jeep gets into first couple gears fine, but starts to slip gears around 55-65 mph. It isnt very "rocky" but it does it very frequently. The transmission was rebuilt a few years back. The "check engine" light comes on intermittently, only during one trip out of about 25, and doesnt come back on during later trips. There seems to be some fluid leak, but is unidentified because our driveway is loose gravel, though it doesnt seem to be oil because the level is always appropriate.