Will shift but feels like it is stuck in park

Has the truck been sitting for a long time? Maybe the brake rotors and pads are rusted shut not allowing you to move? If that isn't the case and this just happened , I suspect the transmission pump has failed. But more diagnosis is in order. To fix you need to have the truck towed to a shop and the transmission needs to be removed from the vehicle to be rebuilt. Good Luck.

As vehicles age, problems occur and things just wear out and change. Often when a car won't go forward or reverse, the problem is just transmission fluid being too low for the trans to work properly, or too dirty for the trans to process it. Check fluid, and if low, top it off to see if that corrects the problem. If not, check fluid and if it's not like new, flush the fluid and change the filter if there is one. If you still have the problem, it might be that the shifter is worn or the cable has stretched; both signs of age. Or it could be transmission internal troubles. Transmissions wear out too, and do it faster if they're not well maintained. The trans might have a worn valve body, defective pump, or seals that are so old/dried that they're leaking. Leaking seals allow pressure to to migrate to other gears. You end up with forward and reverse gears getting the same pressure and neither works. Also check what VRoom said about brakes being locked up.

If your 15 year old truck has over 100,000 miles, probably a transmission rebuild is needed