i was thinkin about running the 75-90 syn. gear oil in my 99 rear end andbody see any problems wit that

 

I would not recomend syn. fluid in the rearend. To thin!

 

Make SURE you do not have an Auburn in there, as there is still some debate as to EXACTLY when GM switched over to Torsen diffs. If you have an Auburn, DO NOT USE Amsoil (or any other synthetic gear oil) in the axle!!
IF you have a Torsen, Amsoil has a new 75W-110 Severe Gear oil available that should work just fine, but being in S.C. you probably could just use the 75W-140 Severe Gear year round with no problems.