well if there was any difference it would probably be a difference in tooth count and wouldn't even mesh up with your pinion....backlash is not the only clearance that needs to be checked when setting up a diff....pinion depth as well as pinion drag should also be checked....but from what i get from your post the pinion was never taken out of the s13 housing so you wouldn't have taken those measurements.....you would have been able to tell if pinion depth was good if you used marking paint to check the mesh pattern.....and that is pretty much my extent of diff knowledge...hope it helps

 

Yea , its just using a s14 ring gear with a s13 pinion, I guess I need to pull her back out and check how its meshing up...