Like most posts, we're talking basic physics - nothing real complex. My tractor weighs about 2500 lbs. Under usual circumstances, I'm in 2WD, such as brush cutting and the like. However, with the FEL doing heavy duty stuff, like moving damp sand to fill in the ruts on my road, then I put a box blade on the rear, and switch to 4WD. I just can't get enough traction for difficult stuff if I haven't counterbalanced the FEL a bit, and have some engine power into the front wheels. Could I get by with 2WD (and with a loaded FEL, locking the rear wheels is not all that useful), yup, but I'd take smaller "bites" of the damp sand, and be a less aggressive in what I do. That said, there's no magic. Last weekend I buried the FEL into the wet sand (it had just rained), and tried to lift the bucket - only to have the tractor's rear start to lift - sort if gets your attention.