Did you check the fluid level in the automatic transmission? Could be as simple as low fluid level. If it's low, this can cause a delayed shift. As mentioned above, how many miles are on this truck? What prior maintenance has been done, such as transmission fluid flush?
You have a warrantee, so I'd say you have no worries. Take it in and get it fixed.
"Never argue with an idiot; they'll only drag you down to their level, then beat you with experience"
2007 F150 STX Supercab (4.6 2V V8), Edge tuner, factory navigation, H-D headlights, Mark VIII electric fan conversion with flex-o-lite controller.