Sorry for this long post but before you just go slapping on a steering controller please heed the following, there are just too many things to consider before jumping to conclusions.

before you go replacing the steering module there are things that need to be known before a definitive diagnosis can be made
- is this a manual steering truck or a automatic guidance steering truck?
The reason i ask this is a manual steering truck has the steering encoder signal going through the dash display module and then in conjunction with the steering module it tells the drive unit which way to turn. With Automatic guidance steering everything is wired differently, it still communitates with the display module but the encoders are connected directly to the steering module.

- have you checked the wire harness cables going through the mast? according to the parts diagram there can be up to 4 cables, one of them deals with the steering system.
- have you checked the output signal from the steering encoder at the encoder, at each point in the wire harness till it gets to it's destination on the tractor unit? (this is the encoder behind the steer wheel/tiller)

-what is the status of the steering module when operating the unit? it has an led power indicator and a status or service indicator on it. If there is an issue with it the service indicator light will be illuminated the power on indicator light should always be on when the truck is turned on. This truck may also have a distribution panel connecting all the components from the platform to the base unit down on the tractor unit, there are many connections there that need to be considered as problematic.

The reason i ask these questions is because a faulty wire in the harness can cause an intermittent signal or a poor signal creating the inconsistent directional movement under a load in the one direction of steer.

One other thing to understand is the steering "feedback" encoder on the drive unit is only there to report the position of the drive unit so it can display that onto the display screen on the dash, it has nothing to do with the speed of the movement of the steering unit. Now the "steering encoder" (on the steering wheel/tiller) is what you need to be checking. This is like a tach generator and it generates a signal in positive signal or negative signal to report to the steering controller on which way the motor needs to move.
If there is a problem with the steering tach generator or its associated wiring that can indeed cause the problem you are having. The manual labels this as an encoder ECR1.
And last but not least the tire itself, if this has a rubber tire on the truck a rubber tire can cause all sorts of issues with steering, the rubber grabs the floor and puts a great strain on the steering motion and if it binds up too much it can make this very thing happen with the steering movement. Now generally you would experience this movement restriction noticed in both directions but who knows, it could be something going on with the shape of the crown on the tire (if it has any crown left on it). The wear of the tire needs to be considered as well) ;o)

So you have to more troubleshooting to do before you can make a decision on what to repair.
So don't just go out and buy a steering module without knowing the full picture first, they aren't cheap ;o)

Good luck