If you can, do the regular checks of the transmission fluid. With vehicle at operating temperature, pull dipstick wipe it clean, reinsert it and remove for inspection. The fluid should be in the 'HOT' crossed checked area and a reddish or brownish color.
If the fluid is okay they your truck likely has an internal transmission failure in the vicinity of the reaction sun shell. This part, with it meshing gear is responsible for reverse and two of your forward gears. I believe you will also find the vehicle only shifts once and will not accelerate like it used to. The transmission will either require replacement or overhaul at a transmission center.