My mechanic just completed replacing the rear differential mounting bushing for $450 complete on my 2007 CTS, 90k miles. I had gotten an estimate from Pep Boys in Pine Brook NJ for about $2K, saying that it was a pinion gear and diff mountings. The symptom was that sometimes when I stepped on the gas, I heard a loud thump. My honest mechanic simply replaced the $40 diff mounting and no more thump!