This is classic and often never properly identified. When changes from toe in to toe out ie forward and reverse, I have found the axle itself is usually the problem with little fix. The axle tilts forward and backwards rocking in the frame cradle. Often caused by striking a tree or other while turning sharp and hitting in a fashion to torque the u channel of the frame open and then the axle will rock for and aft = toe in toe out as each tie rod is moving the alignment in accordance with the distance change. I have not found a good fix. I suspect a hard fix might be to shim the axle in the frame or weld it as a non pivoting front axle if your ground is fairly level but quality of cut will suffer as well as traction and divots. All depends on your need to continue it's use or abandon it. Sorry for the bad news on this design and there are many units with this design under many makers.