If the fluid is full and the transmission is slipping it's time for an overhaul.That transmission you have in your truck will be very expensive to repair if the overdrive unit is bad.This info is for an automatic if you have a manual transmission you need a clutch and should replace the clutch master and slave along with the clutch.
Master ASE Certified.L1 Advanced Engine Performance Certified.Pa Emission Repair Technician.Just Answer Car Category Mentor.