can you push the jack with the battery plugged in and key on, or does the brakes hold it in place?
reason I ask, I think this jack has a plug in the harness, to release the brakes, but it won't drive with the brakes released with this plug.
There is a proximity switch at the base of the handle,there is a red led on it that should light up when it sense the handle in the down position,if the led lights then it is more than likely the tiller card in the top of the control handle where the buttons and fwd/rev is....
sounds like switch at base of handle, brake release switch (which also turns on controller drive circuit).
knowing serial number will help though
if machine has tiller control it could also be that being bad not activating system drive.
if has tiller control watch the traction controller when you turn handle , should be an LED on it blinking... if anything is wrong it will blink a code.
What's the serial number of the truck?