Are both drive wheels locked together when turning left or right making steer tires slide? If tractor turns OK in both directions diff lock is not working, may have been disabled. If tractor does not turn OK diff lock rod/fork assy may be stuck in on position. If you have early 4 bolt diff lock control housing on axle housing remove assy and see if you can move large rod with coil spring back to left/disengaged position. Cam in box assy pushes rod to right to engage diff lock, spring should push left to disengage, may be rusty,stuck in on position. Later model MF tractors used large external rod that pedal pushed in to turn on, not sure which one you have.