2002 Jeep Laredo Transmission will not shift from 2nd to 3r

About 1,500 miles ago (86,500) I experience transmission not shifting from 2nd to 3rd with my 2002 Jeep Grand Cherokee Laredo. The problem was not consistent as sometimes it shifted sometimes not. The check engine light also came on. The last transmission fluid change was at 50,000 and again (at the 50,000 mile mark the fluid was a little low too- but not low now), but no leaks were detected then or now. I had the fluid changed at 87,000 and the problem stopped including the check engine light went off.
Now after 1,500 miles same problem again of not shifting from 2nd to 3rd gear, but this time it occurs consistently with the check engine light staying on.

What do you think the problem is and what should be done? Thanks for your opinion