That's not always the case. Tires are a huge part. My 93 Dakota was absolutely useless on snow when I first bought it. Even with 200+ lbs. of weight in the back. When it came time to change tires I got some snow tires put on and you couldn't hardly stop it. I had it lowered 2 in the front and 3 in the back and I could stop in snow up to the grill and take right back off like it was hardly there. Had a buddy that had an old International 4x4 and I could go places he couldn't with his locked in 4wd.
As for the original question. I find no difference over the axle or at the back end when the weight added is just 200-300 lbs. If you stop and think of it adding 200 lbs. 2-3 foot behind the rear axle can't affect the front end very much cause the front end is so much heavier. I doubt 200 lbs. behind the axle would take 50 lbs. off the front wheels. Just not enough leverage there IMO.