swoop, if you jack up the truck, then you don't have resistance against the drive motor, even if you are in 1a. (and a drive motor stalled dead stop for more than 3 to 5 seconds with full battery voltage is likely to have armature [and thus the commutator bars] expand [raise] so that the brushes jump in their holders as it runs, which is what causes a lot of electric forklift drive motors to sound like they have "cards in the spokes of a kids bicycle". if you can not work the hydraulics, then a load bank from a battery company is the answer to test the battery voltage drop under load.
my "W.A.G." money is on a bad connection right at the plug connector to the controller, since he gets no error codes, but no operation and has signal up to the controller (I ought to look up or ask what controller is fitted in this particular unit)