ok after carefully looking over the pic's mike sent this is my observations
- the box on the left side IS in fact a resistor pack (i think most of us agree on this now)
- the component (circuit board) on the right side i'm thinking is a steering interface board MAYBE combined into a drive contactor control
- the accelerator control even though it has the array of switches on it there was also an integrated circuit board he had to remove to show us the switches that also interacts with the contactors (could be interfaced with the steering control board.
- and also as MEngr pointed out, the 4 switches around the steering position indicator component in the middle of the truck panel area also may be an area of concern. check the wires from the switches and trace them, see where they go, if they go into that board on the right side then that confirms my suspicion that it may be a steering control board. If theres a problem with this board your contactors may not pull in correctly or at all and it very well could effect how the motor speeds for each side react.

As i pointed out before, check all the switches in the accelerator and make sure they are working properly, don't just listen for a click, use a VOM meter and check them. Do the same with the switches on the steering position module as well. If those steering switches stop working or get out of adjustment it will make the motors act up or not work properly.
Now as for that circuit board on the right side (what i'm calling the steering interface board) it looks to have several relays on it? there may be an issue with that but i'm not really sure how to check this without a wiring diagram of some kind. Like what was suggested, you may have to send it out and have it checked and/or repaired.

Of course this is all speculation on my part making educated guesses as to what these components are, without a manual i have no way to determine what they call these components. i do have a contact at linde and i will send him an email and see if he can provide me with any information that could be helpful, no guarantees on this but i will try.