First, what kinda truck is it?

Second, put the truck on the ground and hold the spur gear in place with your finger. Push the truck backwards to see if it wants to try to move the gear under your finger. It should as long as nothing is wrong with the differential. Also, make sure the clutch bell spins freely and the gear mesh is correct. Sometimes a hard landing and loose engine screws can cause the engine to jam into the spur gear and lock the two gears together. Also, when you try to drive forwards, does it sound like the engine is straining or is it just free revving and nothing is moving, like it's in neutral or something.