I hear this is a common problem, so here is a summary of what I did.
Our 1999 Grand Cherokee (WJ) was shifting violently from gear to gear, and the check-engine light came on at the same time. Doing the ignition on-off x3 trick, the instrument cluster spit out the code "P0700". We ran over to our local auto parts store and borrowed an "OBDII" tool and pulled the following codes out of the computer:
Check Transmission DTC's
This code indicates that the EATX (Transmission controller) has an active fault and has illuminated the MIL via a BUS message. The specific fault must be acquired from the EATX via a DRBIII tool. (Input or Output Speed Sensor failures often cause this code and are a common failure part on WJ Grand Cherokees).
Input/Turbine Speed Sensor Circuit
Gear 1 Incorrect Ratio
Gear 2 Incorrect Ratio
Gear 4 Incorrect Ratio
I found these translations at WJ Jeeps
and the code P0700 had a link to another page titled "Transmission input & output speed sensors"
Now I was pretty sure the input speed sensor was broken because code P0715 indicated the "Input/Turbine Speed Sensor Circuit". We then went to the local dealership (because no one else had the part in stock) and purchased this for $43. Went back home and followed the steps listed on the WJ Jeeps page
and 30 minutes later we were finished.
I cleared the fault codes using the OBDII tool, drove around the block a few times and tried to pull the codes again but the tool simply said there were no codes to pull. Very simple, very easy to do...one bolt! I hope this saves you a trip to the mechanic.