i believe it's supposed to go like this
when fwd is selected both RF and LF contactors pull in during straight fwd travel.
when steered 90deg to the left LF drops out and LR kicks in to reverse the Left motor.

and if steered to the 90deg right then the opposite would happen RF contactor would drop out and RR would kick in.

if you were traveling in reverse the opposite would occur.

Now being this system has a potentiometer on the steer assy in the rear i would pay attention to the signal from this to the steering controller. Since the steering controller tells the contactors in what mode they are supposed to be in when in straight ahead and in turned conditions.

Do you have a service manual?
your going to need it to diagnose this problem effectively.
too many things going on at the same time to just be guessing at it. :o)