First thing I’d check is the fluid level and also condition. If you are unsure if it was ever changed, might not be a bad thing to get it done. Don’t forget to change the external filter. No trouble codes either? When I got my truck that was the first thing I changed, but the shop didn’t quite fill it up to the full marks, they filled what was specified. I have a deep pan which requires more fluid. Anyway, I had a slip happen from first to second gear, especially when cold or on an incline. Turned out it was just low enough on fluid to cause problems. No issues since filling to proper level. Hope it gives you somewhere to start and is an easy fix.
2008 LMM 7”FTS LIFT 35’s on 20” wheels. All stock under the hood...Thanks California