I'd bet its a worn out oxygen sensor. Especially if your van has 100K to 120K miles and it's never been replaced. The car's computer is supposed to store fault codes in memory for a certain number of ignition cycles after the Check Engine Light (CEL) is triggered. You need to get the code read within probably 20 or so trips of when the CEL was triggered, or the fault memory will be cleared until it is triggered again.
Speaking from 13 years of Mopar minivan ownership experience. A couple suggestions:
1) Your 1994 is "pre-OBD II" standard, so you should be able to read the codes yourself, with what Mopar guys call the "key dance". Basically you cycle the key on and off 5 times in quick succession (without starting the car) and the CEL will flash out a series of pulses that you can translate into what CEL code is set.
Here's the specifics of doing the key dance and interpreting the codes.
2)
www.allpar.com is the "CPF" of Mopar. Go there and browse the tech help section and search the archives of the message forums. I am positive you'll find a wealth of information that's worth spending the time to look thru. You'll also find all the eccentricities, common problems, and solutions pertaining to these hearty vehicles.
2) Alternatively, go to one of the chain store (Pep Boys / Precision Tune / etc) and they will hook it up to their diagnostic machine for free. They should be able to read the code for you and give you an estimate on the spot.
3) While you're there, pick up a Haynes repair manual and you can read up on all the codes and what they mean. The book is pretty well done and is definitely worth the $15 investment.
Good luck,
Dave