I doubt it's the card too. The first post said it did hum and your next one said no oscilation. That would sure narrow it down if it is or isn't. You said when you tested the pot, you pushed the lever. If reverse is the problem, doesn't the pot spin the other way when the lever is pulled for reverse?

ok...
not a contactor issue
but i doubt its a card issue if forward is working perfect
if your getting just a hum like the system is pulsing in reverse but just no increase in oscilation when the pot voltage changes then it must be either the range on the pot or something else related to the reverse circuit. You said new contactor? Are you sure the power circuit was put back exactly the way it was when it was reassembled? Could it be possible one lead was put on incorrectly on the reverse side?

I'm just guessing here but it sounds like something that was mistakenly done during a repair job maybe? Go back and retrace the steps made and insure EVERYTHING is wired up correctly.

brand new contactor ***'y and no brakes are dragging -the issue is no oscillation in reverse. Anybody else? Next step is a card.

yes check the contactors, if they are worn they may appear to be making contact but might not be making enough to make a good connection.
I'd also check the full range of the voltage readings comming from the potentiometer in both fwd and rev and make sure your getting full range of motion in both directions.

Also make sure there is nothing keeping the truck from moving in reverse such as brake dragging. Sometimes brakes can drag and you will get movement in one direction but not the other.

Does steering either direction affect your travel? No reverse with contactors closing and your "pot" decreasing from 4v to 0v indicates a contact tip issue. Rarely do both reverse contact tips fail at the same time.