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.
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2005 F150 STX Regular Cab (4.2 liter V6)