The rearend you have is either a 7 5/8" or an 8.5" 10 bolt. If you have an automatic, you have the 7 5/8" rearend, probably with 3.42's. If you have a 5spd 4.3, chances are you still have the 7 5/8" rear, not sure when they started putting 8.5" rearend's in the 4.3 5spd trucks, it could have been 1998. 3.73's would be a good upgrade for the automatic, I wouldn't go anything more than 3.42's with the 5spd though, especially if it's a daily driver.