auto trans on cummins 5.9L won't upshift on 2000 Dodge Ram 2500

Tranny has been shifting fine, no heavy pulling just standard driving. started 2 days ago. place it into drive and it was slow out of neutral when cold. after warmed up, it shifted fine. As of this morning it goes into drive and reverse gear okay when manualy shifting but will not upshift to 2nd out of drive under standard driving conditions. Fluid has been overfilled at some point.

sounds like seal failure in the trans. take it to a trans shop for a bid. will most likely need a complete overhaul