If you got 4WD engaged that can help in those situations, for sure.
But you can get in that exact situation if you *don't* have four wheel drive as well. The thing to do if it happens on a 2WD tractor (which includes 4WD, but not engaged) is to push in the clutch! Then pump the brakes like you would in any sliding/skidding situation. The trick is to try to get the rear wheels turning enough to straighten out the tractor - while keeping your speed as slow as possible, of course...