GMC Troubleshooting Questions? Ask a Mechanic for Answers
I am sorry for the long wait and that you are having this problem
is the transmission physically being shifted into drive ? and it just acts like neutral ?
is this a 4WD ?
with reverse working fine we know the fluid is full and the transmission pump is good and making good pressure since reverse works
with it not moving the truck forward when forward is selected is going to be an internal failure going on of the forward clutches or possibly the one way clutch inside the transmission
the shift solenoids or valve body would not cause this problem but something inside is slipping or not being able to apply a forward gear
I am sorry to say this transmission will need to come out and apart to be repaired and usually ends up being a rebuild to correct this issue
a lot of times it is a lot cheaper to just pick up a used transmission for a junk / salvage yard and just swap it out into your truck