My brother owns a 1999 Chevy Silverado 4WD 1500 Z71. According to the engineering codes, it has 3.73 rear end gears with no posi. He wants to upgrade to 4.56's, and I wanted to ask here to make sure we have things straight. As I unstand, he has a 8.625" 30-spline 10-bolt rear end, but he can buy an 8.5" set of 4.56's and that these will work. Also, because he has four wheel drive, won't he need to buy two identical sets of gears, and simply put one on the front and one in the back? Lastly, doesn't he only need one posi unit? Thanks for helping us with this. All I have to say is that I find my f-body much easier to work with.

-Justin

 

The 8.5 4.56's will work fine, I just put a set in my truck. As far as the front goes I am not sure if it is an 8.5/8.6 or not. As far as posi units go he can put one on each end although the manufacturers never offer this for fear of the front unit locking and causing steering problems.