search eBay for "8.8 limited slip" for the cheapest price. I just bought a Ford Racing L/S unit for my truck from Summit Racing for $215....
This is the 8.8 axle unit:
This is the 7.5 axle unit:
you have a 28 spline axle. The best way to tell is to look at the tag on the drivers door or door frame. there is a spot that says "AXL or AXLE" and right under that will be either 2 numbers ( like 96) or 1 letter and 1 number (like R6)......when you find that look at this list i put together for the site and find your number. It will tell you what axle you have and your gear ratio....:
buy as far as really what gears you should get, use this formula:
new tire size/old tire size x stock gear ratio = new gear ratio (now you may have to either go up or down to the next gear they make for your truck since they dont make every gear ratio possible)
for example here is what mine would look like from stock to the 35's i am running now:
35/31 x 3.73=4.21 ......since they dont make a 4.21 gear, i went with the next lowest gear which was a 4.56....