Single speed or 2 speed?
Slipping only in Forward, or slipping both Forward and Reverse?
Slipping could mean that you actually have a loss of oil pressure in the clutch application circuits.
The most common cause of SLIPPING in these transmissions is a sticking, or non returning inching pedal cable, which causes the application oil pressure to be dumped from the clutch supply circuit.
Take the floor board cover off and push down hard on the left brake pedal (inching pedal). See the bell crank lever on the transmission that moves when the inching pedal is pushed down.
Now, let up on the inching pedal and put your hand on the bell crank lever (inching valve lever) and see if you can push the lever further to the released position than it was able to reach by itself.
If you can push the lever further to released mode with your hand, the inching cable is sticking and preventing the inching valve from deactivating.
If you are able to move the inching lever more off with your hand, drive the truck without touching the inching pedal and see if the slipping goes away.
If so, replace the inching cable.
Inching cable Toyota part number 47110-23331-71
There is also a coiled torsion spring on the inching pedal pivot hub that helps to lift the pedal back to the returned position. If that spring breaks, the pedal will sag and keep the inching cable from returning completely.
So, check both those items.
Toyota part number for the torsion spring is 47119-23331-71