It was a posi, i found with that particular setup that i had to actually apply power to the rear, with the brake on to i guess "lock" the rear end, i know it isn't fully locked, but it seemed to work better when launching, if i just gunned it without doing this, it would just break one tire loose.
Your traction problem is definately caused from the open diff, get yourself maybe a loc-rite, for positive locking action, as the posi will still slip at times.
I raced a "stock" 90something Toyota pick up tha was hangin with me on my rear quarter all the way down the track, couldn't figure it out, until he opened his hood, he told me everything was "stock" well it was, there was a factory supercharger mounted to the stock v-6. His only problem was the open diff, otherwise i believe he would have beat me, that would have really sucked too.
The other thing, i never do long burn outs with my radial tires, i just scrub off any debris, spin them a couple of rotations, i know, it looks better to smoke'em, but have seen that it's not good for the traction to do this to them.
