If you run out of traction when the box fills up, try pulling up on it with the 3pt. Don't lift it, just pull up on it a bit. If you've got a 700lb box of stuff and you pull up on it by 500 lbs, the box doesn't come off the ground (so your material stays in) but you put 500 extra pounds worth of traction on your rear tires and can often continue moving. I've found you can actually lift a full box maybe an inch off the ground and 95% of your material will still stay in the box, but it gives your rear tires more traction and slides easier over the ground. In the pic I posted earlier the 60" box is full and I was pulling it across the ground in that position. Tilting the box back with a hydraulic top link also makes it drag a bit easier when full.
