I have a 94 Grand Cherokee. The check engine light came on last night. I've checked the oil and its good. The Jeep is running fine. Any ideas on whats going on?

I think you can get your codes using the key on, off method. Try this link and let us know what codes you come up with.
http://www.allpar.com/fix/80s-codes.html

what size engine do you have, here is another link as well.
http://minimopar.knizefamily.net/fault/index.html

If there's an Autozone similar store near you they can scan the codes for free. That might save you some research.

unfortunately no parts store can scan you jeep, because it is a 94, the only place to take it to get it scanned is the dealership which will cost you.

You can extract the codes yourself by following the links above or here it is.....

Turn the key to the ON Position, then OFF, then ON, then OFF, then ON - leave it in the ON position this time.
The CHECK ENGINE LIGHT will begin to flash the codes, they will flash in double digit form with a SHORT pause between digits and a LONG pause between codes.
example:
flash pause flash flash - 1 pause 2 = 12
pause pause pause
flash flash flash flash flash pause flash flash flash flash flash - 5 pause 5 = 55

Code 12 - Battery was disconnected within the last 50 ignition cycles.
Code 55 - End of fault codes.

On vehicle equipped with digital odometer the code will appear in the odometer (example: P0622)

just post your results and we will tell what the meaning of the codes are.