XWD Differential Problem
I have the exact same problem with my 9-5 AWD. The differential seems to apply the brakes when you accelerate. Pull the two ABS fuses under the hood and see if the problem goes away. In all my in tenet research over the past tweo months I beleive the poroblem is with the haldex Generation IV AWD. Specifically the DEM which electronically controls the limited slip differential. I changed all the fluid a couple of years ago but am willing to change it again based on some other post. I think the unit needs to be removed and sent to a shop which fixes the same units on Volvos. Let me know what you think.