[Duckjet]

Need some help troubleshooting.
2005 325i 150k. Drive train harmonic rumble at 50mph plus, completely drops out below 50 mph. My wife doesn't feel it but I do. Drives me nuts. Here is what I have done so far.
1) New Driveshaft assembly carrier bearing, flex disc (had a very stiff rear u-joint and vibration at 35mph) Fiex disc is installed correctly.
2) New drive axle / CV joint assemblies
3) New Front rotors and wheel bearings.
4) New rear wheel bearings and hubs
5) Tires and rims switched out with a known good set.
6) Differential fluid checked and topped off.
7) Had the folks at Simpson Motorsports (BMW Specialists) in Iowa City take it for a spin and put it on a lift, they weren't seeing anything obvious other than the rear spring is broken (very first lower coil) and I'll be replacing.
Engine is smooth as glass slowing revving in neutral.
It's a steady rumble, doesn't matter if power is on, off or in neutral or brakes applied. Something in the diff? Bearings? I'll probably go ahead and replace the rear rotors even though I don't believe that is the issue. New drive shaft possibly out of balance? Open to any suggestions. Thanks.