Ohh...you mean the Dana Spicer Trak-Lok
It's a LSD (limited slip differential) or "Anti-Spin Differential" it's called from the factory
It got it for $259 SHIPPED from
www.4wheelparts.com
Part number: 80754
I got it sitting here, but still waiting to put it in along with my 4.10 gears
Should have it done within the next month I'm hoping...just need some time (and a day off)
Paying a shop to do it though...$300