only in reverse??

This is one of those questions that get you going... like, my 92 powerstroke, it is 4 wheel drive, but when not locked i can put it in second and spin the one wheel wonder, and it leaves rubber, but if i lock the front and put it in 4 wheel drive, I leave 4 strips of rubber, the front is posi but not the back... Crazy stuff right there... haha

Im not too credible, but I assume it is because you are going in reverse and moving in a direction opposite to which has all the weight so your tires can spin better, because it isnt pulling forward... I could be way off, but at least I tried...

It's mostly luck. I have left 1-2 really wicked 2 wheel burnouts. It just depends on the exact conditions, and even what you have in the trunk. It may also have to do with axle wrap unloading the wheel with more weight and allowing it to spin both tires more easier. I know that in a forward gear, it tries to lift the right rear, and that corner has the least weight on it to start with, so it will try to spin before the other tire loses traction. I suppose in reverse, the left rear would try to lift up, and it just happens to lose traction at the same time as the right side.