stall is when your engine is turning, NONE of your wheels are, and you're in 4low, 1st gear. There's nothing wrong with the transmission, it's just that it is simply unable to transmit the amount of torque you're asking of it.

It happens quite a bit when you have lots of traction but you're on a serious ledge. A couple of weeks ago I was 'wheeling with Lordtrunks and stalled, all four wheels had tons of traction but the front wheels were still climbing a couple of ridges and the rear wheels had to climb about 1.5 feet sheer vertical. Since the truck could not move forward AT ALL until the rear wheels came up a foot and a half, I was essentially hooked in place. Even with my 2.48:1 t-case, no forward motion was going to happen. I ended up backing down, and taking a scarier but less vertical notch to the right instead.