I have this exact problem on my 87 745 manual trans car, driveshaft knocks for a second or two on accelerating from a stop. All the wear items in the driveshaft are new and there is no really notable vibration at any speed.

There is an old TSB on shipping the center support bearing up or down, this was a problem known when these cars were new. I suspect it has to do with it being a mobile rear axle instead of a fixed center section like most RWD euro cars with two piece driveshafts. It may well be related to worn springs and altered ride height and/or worn bushings. But it's defenantly the center support banging around for a second or so on takeoff. If you reach in and grap it by hand and yank and push hard, there is quite a lot more movement in that rubber support than on the Mercedes and BMW's I've had, presumably there is a reason for this. I've had people that used various methods to stiffen that rubber up tell me it had adverse effects at other driveshaft speeds, and one account of an experienced driveshaft shop saying it was probably just going to be that way to some degree from the design. I really wish they had just put a simple single piece shaft in there myself.

As for me, I haven't messed with it yet. All the parts are new so I'm reasonably sure it's not going to come flying out from under the car, and I'm hesitant to make adjustment with shims or such to the CSB as it might be caused by worn rear axle bushings or springs. It's liveable, annyoing, but liveable.
For now. Call it another silly Volvo quirk.