Depends on year of truck if it doesn't have over drive and even then maybe there is a vacuum line going to shift module on trans mission check and see and if so see if it came off at transmission and if not trace it up to motor
It sounds like the VSS (Vehicle Speed Sensor) has failed. If the VSS fails, it can cause the symptoms you are describing. Check the wiring and make sure your VSS is properly plugged in before you...
Does it shift out of Park?
If not check stop light fuse - shift interlock and stop lights are a common circuit
If it does shift out of Park then shift linkage may need adjustment
Try turning key...
It could however it is doubtfull. If the transmission fluid were dangerously low, the vehicle would also exhibit other driveability problems such as slipping. You can check your transmisison fluid by...