I have had a similar experience that I easily fixed: When in 2WD my Gen 1 rode straight. When in 4WD it would pull to the right AND jump in and out of worn ruts in a highway. The fix cost nothing...

I checked the inflation of the tires and found one (on the right) to be low. After inflating all tires to their proper pressures, the pull to the right is gone and the tendency to jump in and out of ruts is diminished.

Why is this? Because in 2WD the four wheels are free to spin independently of each other. Therefore an under-inflated (slightly smaller radius) tire is free to spin at its own slightly faster rate. In 4WD the four tires' rotations are "locked" together. Any variation in tire diameters caused by either pressure variations or by using one new tire (say a spare) with three worn ones will cause pulling in the direction of the tire with the smaller diameter.