this truck have the cableform contactors on it?

try this trick and see if it just needs a new contactor kit
jack the drive wheel(s) off the floor
activate the drive in either direction and then wiggle the direction contactor that is pulled in and see if it takes off running. These contactors were well known for failing even though they did look ok and appeared to pull in. The armature of the contactor can get worn and hang/bind up.

I would also check the accelerator pot with a vom and make sure it has full range.
And of course double check the motor brushes, grab the leads and pull them out a bit and let them pop back in place, this will tell you if they are worn out too much or are hanging up in the brush holders. And like kevin says, look at the condition of the armature and make sure its not arc'd up and damaged. A good armature surface will have a smooth surface and be a chocolate brown color or shiny copper where the brushes ride.