I have a performance cars venom 427 (similar to backdraft). I have the thunking in the same place, when I am starting in 1st from a standstill. Really have to go easy on the gas when letting in the clutch to get a smooth takeoff, almost letting the engine stall before I give it easy gas, or she bucks like a bronco.
From what I have been told, this sort of thing is usually related to the improper alignment of the driveshaft relative to the differential. In the cobras, our driveshaft is so short that any small angle can make a big difference.
Running 26psi instead of 34 in the rear fatties helped absorb the shock a tad. I may also try some red line anti-shock differential
oil. Other than that, I think I need to fiddle around with the tranny/engine mounts to get the angles perfect.
I have a 2001 Mustang GT, and it has wheel hop absorber units mounted behind the differential at an angle to the rear frame. Very similar to the setup that ERA is currently testing for the Jaguar IRS in it's cobras. That would probably help this problem, but thats a bit of a design effort for the BMW rear that I have.