Change the tires if you have the originals but if not, don't worry about it. They probably would not notice the slightly larger tire size, and it would be pretty tough for them to claim the slightly larger tires would cause the transmission to slip when it's a common and well known problem.
Since you are covered under the extended warranty, just take it to the most competent dealer you have in town and let them fix it. They will probably just replae the solenoid pack and the valvebody.
If it has been slipping for a long time, it may have damaged the bands and clutches and will require a complete rebuild. So If they only replace the solenoids and/or valvebody, test drive the truck carefullly and be preparted to take it back for more repairs if it still slips even a little bit, especially the 2-3 shift.
...Rich