What is wrong with my truck?

Hello, the vehicle goes forward normally with no concern? That's the piece that doesn't make a whole lot of sense. Low and reverse gears share a band. This provides engine braking in manual first gear. Does manual first gear work? You also have a reverse clutch that applies the input shaft to the reverse sun gear. Basically -- at the end of all this, internal mechanical concerns are likely the cause of what you're experiencing. A tear down and repair of the transmission appears likely needed, but have hands on testing done at a shop near you. www.repairpal.com/repair-shops