where abouts do you measure a diff to see if it is 2wd or 4x4?

 

You can see from the top mounting bolt. On a 4x4 diff (7") it's one long bolt going through from one side to the other. On a 7.5" diff (2wd) it's two short bolts that bolt into the diff. So in that case the holes in the diff will have threads for the bolts.

 

On the 7" version, that doesn't tell you if its a a 4x4 LSD or not. It could be a standard open diff for any of the other cooking Sierras.

 

The same goes for the 7.5" diff, it's also found on standard Sierras and Granadas. But the question was not "how can I check if it's an lsd?".

 

If its a front diff it's definitely for a 4x4.....