Welcome to TRS
All automatic transmissions run on Fluid Pressure, just FYI
They have clutches and brakes(the "bands" are the Brakes) inside, clutches are engaged and disengaged using pressure, brakes are applied using pressure, gears are selected/shifted using pressure.
Front pump(main pump) provides that pressure, it is at the front of the transmission and is connected/driven by the torque converters rear shaft, so pump RPMs match engine RPMs, exactly.
Reverse requires the highest pressure, 150+ PSI, if there is an internal pressure issue slow to engage Reverse(or no Reverse) will often be the first sign of that, also higher RPM shifting or slipping.
When you raise engine RPMs you are raising Front pump RPMs which increases available pressure in the transmission, which is why, in your case, Reverse then engages.
Valve body routes internal pressure via passages and valves, so yes there could be gaskets or seals that are leaking pressure internally so pressure is lower than it should be at idle or higher RPMs overall.
But it is not a "for sure" fix, it can be done in the truck so probably worthwhile to try, if trans still has issues then it will need to be rebuilt but the new valve body can be used so not a waste of money.
The sooner the better, low pressure causes clutches to slip and bands to slip, this causes alot of wear on the friction materials used in these parts, like riding the brakes on a vehicle or clutch on manual trans.
Welcome Visitor! Please take a few seconds and 
