Hello and welcome to just answer
The problem you are having is in the transmission
It will be one of two things 99% of the time it is the govner pressure switch and pressure transducer that need to be replaced. This involves removing the transmission pan and a couple bolts that hold the sensor and transducer in place.
The other 1% of the time it is a bad valve body in the trans that has to be replaced.
I would start by replacing the sensor and transducer first, because it is alot cheaper than a valve body and it will probabliy fix the problem.
Two parts the part #s are 56028 196AA transducer and(NNN) NNN-NNNNsolenoid. These are the parts to put on first.
That is not alot of miles for a truck as long as the trans has been serviced.