I have 99 GMC Sierra 1500 4x4 (new body) and the front diff started making a clicking noise this past week. I have confirmed that its the diff and not the axle shafts. My question is this, My brother-in-law has a front diff out of a 99 Suburban but he is unsure what the gear ratio is. I can't seem to find what was offered in the Suburban for 99. I- like most of the country am out of work so I am trying to cheap out by swapping the diff., but I need to know that it is a 3.73 ratio. How can I tell? Any help, as always is appreciated.

 

...something about jacking up the truck, see how many rotations of the shaft it takes to spin a wheel a full rotation, or possibly the other way around, haha... sorry, I'm just confusing myself the more I try to remember...
But with the results of that you can tell the ratio...


Also, just curious, how did you narrow it down to the problem lieing in the f. diff?

 

First, the noise was most noticeable in reverse. So I jacked up each front wheel individually and carefully went in reverse and still had the noise.At this point I was thinking it was the axle shafts. Next I jacked up the rear end and put the truck in reverse and yep still had the noise. So I removed the front drive shaft and then tried reverse again with the rear end jacked up-no noise. So I turned the front diff. by hand and I can hear the clicking and feel where it is hanging up.