BMW Repair: high speed vibration, speed vibration, rear rotors

Question
QUESTION: I have a 2004 BMW 545 with a manual transmission. The car has had a vibration that can be felt in the seats and the shifter when cruising at 65-70 MPH.There are no noises associated with the vibration and it occurs in any gear at that speed range. It also occurs when coasting through that range in neutral. Above or below that speed, it is smooth. I have replaced many components at the advice of BMW technicians, but the problem is still there. I have installed 4 new rims and 2 sets of tires. Had the wheels balance multiple times with a road force balancer. I recently replaced the rear rotors and parking brake shoes with new return springs because they were hanging up. I have inspected the exhaust and it is tight. The motor mounts and transmission mounts visually appear ok, but have not been replaced. Could this be due to a bad axle shaft or be somewhere along the driveshaft? The car has 67,000 miles on it. Any advice is appreciated as I am at my wits end. Thank you

ANSWER: I believe you need to check the center bearing.I have seen the metal brackets actually crack and break.

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QUESTION: Thank you for your quick response. Regarding a center bearing. Wouldn't that cause a noise if it were cracked? The vibration is not tremendous, but is evident and annoying. Enough to vibrate the seat backs and the shift handle. Also, it doesn't matter how much weight is in the car. If you do feel it is the center bearing, should I replace the whole driveshaft or just the bearing. Are any special tools required to do this or is it straightforward? Thanks again.

Answer
The center bearing is enclosed in a rubber/metal housing.The metal part seems to crack although the rubber is intack.The only way you can determine that it is broken is by physically checking which requires you to lower the the exhaust shield.To replace it it doesn't require any special tools,just patience and time as you have to remove the exhaust and the driveshaft.It is pretty straightforward to replace. The bearing assembly isn't really expensive and the driveshaft doesn't need to be replaced.