I think you may have the problem with the symmetry adjustment of the drive transmitter. The adjustment may be done by turning the rear and front bushing (in travel direction) of the transmitter device.
It's not so easy to descibe it without drawings.
The drive transmitter - it' the black box on the top/rear of the front axle.
I has two valves stretching on sides and the small axle linked with the lever towards the back of the truck. There are (on the axle) the bushings to be adjusted.
Good luck
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