The band might need to be adjusted/tightend. Get a torque wrench and get under neath the truck. On the drivers side of the tranny where the kickdown/shifter controls are there is a band adjusting screw. Theres a lock nut and then the screw, losen the lock nut and tighten the screw to 10 ft-lbs. Back off the adjusting screw exactly 1-1/2 turns. Hold the adjusting screw from turning and snug up the locknut. See if it still slips. Get back to us. All information is from 365-127-79A Ford Truck Shop Manual.

I had a hard time finding my locknut/screw, was covered in dirt and looked like it was part of the transmission housing. It should be about 1"-2" directly infront of the kickdown/shift controls are.