Probably a brake. Make sure they all turn if not check to see if the thing is stuck in two gears. Pull out the shifter and look for the the shift slots. Make sure they are lined up. I've never seen this in a truck but my brothers WD tractor gets stuck in two gears sometimes. Will stall the engine faster than snot. Happens becasue the shifter is wore. Take out the shifter re-alight the shifting slots and start driving. Though if this is the case you may want to get it fixed. LOcking the trans in two gears going 55mph would likely be really bad.
