99 Mustang GT Differential
 

I have a 99 Mustang GT. I believe the differential is shot. There is oil everywhere and a horrible whirring noice (starts at about 35mph, goes quiet around 80mph (can't figure out that one)). I live in Napa, Ca., anybody have an idea of how much this may cost me to get it repaired and any suggestions on where to have it done? :confused: