I had the same problem, changed the speed sensors, but it still occured. You can try the speed sensors, there's 2, $100 for both, but after I did the sensors, I ended up plugging in a scanner and finding codes, pm me for the exact code #'s but the codes said lack of power to tcm, which could be a short in the solenoid, and the others said low tranny fluid line pressure, which could mean plugged filter or line. Seeing as this is a notorious problem on wj's, a mechanic buddy of mine threw a new solenoid unit in his daughters wj and it was fixed. Another thing may be the tcm/PCM may need a flash
Lake Tahoe, NV
"There's no JEEP like a SNOW JEEP!"