While preparing my 745 for trailer towing, I got inspired to replace my rear springs with IPD overload springs, mostly because the rear of my car has been kind of low for a while, and I knew the tongue load would cause the driveshaft to rumble on takeoff. I was pleasantly surprised to find that the springs fixed my rumble problem, whereas the fix describled in the Volvo Tech Service Bulletin (couple of fat washers to jack up the center driveshaft support) had not had any effect.

Anyhow, if you have the "rumbles on takeoff" problem, I suggest you have a look at your rear springs before you go the TSB's route. You might just have a sagging rear end. No offense!