1999 3/4ton 4x4 with Cummins. Auto transmission.

Was out pulling a small load the other day and after the first stop went to move again and it was in 3rd gear. It will not shift out of 3rd even if I come to a complete stop. If I put it in range 2 or 1 it will downshift to 2nd gear, but not 1st gear. Overdrive never kicks in and as far as I can tell the torque converter isn't locking up at any time either.
I unhooked my trailer and let it sit overnight and it still wants to stay in 3rd gear all the time. There are no check engine lights or anything on. Checked fluid and it is fine. Any ideas?