Check the TVVA motor on the side, make sure it is working. You can be under the truck and have someone open the switch. The motor will cycle as it always does when it starts.
You can do a pressure test. The front port on the transmission nearest the dipstick on passenger side put a 200 psi pressure gauge on it and see if there are any differences in the pressure when it shifts fine vs when it does not. Pressure with stock
VB should be in the 55-60 psi range @ 45 mph. 100 psi or more with modified
VB.
You do not mention if the
VB has a shift kit, is from a builder or if it is stock. Sounding to me like a
VB problem or bad worn seals on the pistons as was told to you earlier. I have seen VBs with shift valves getting stuck or with worn bores.
I should also have asked, How many miles on this truck; transmission? Transmission due wear out at some point.