Well, I've read quite a bit on this, but I'm curious to see what your experience has been when an 850 turbo makes a harsh thump while shifting into reverse. I know I'll get scolded for this but the fluid is sitting almost an inch high on the dipstick when hot, but I haven't been able to get it drained just yet. Could this lead to the thump? It's actually more of a bang. Thanks for your thoughts on this

 

Have your Universal joints on the drive shaft checked.... ;-)

 

The CV joints in the front shafts could be the problem as sometimes they will make a more noticeable noise going in one direction than the other but usually they will click and more when turning. What you describe would be something more common to an all wheel or rear wheel drive with a drive shaft and actual "U" joints.

You should also check your motor and trans mounts. The upper mount on the firewall offset a little toward the driver is know to work it's way out and that could also cause a bang when shifting.