rumble in the drivetrain

I finally threw enough money at my '89 300e to get rid of all the exhaust, brake, and suspension noise but now I'm hearing a very slight noise and more
noticeable vibration coming from the drivertrain. Feels like a tire out of balance but the tires have been recently balanced so I'm pretty sure it's not that. I did replace the front flex disc and the rear disc looks good. Just replaced the brake pads and discs and shocks so I'm not thinking it's coming from there either--had everything aligned when the tires were balanced.
The vibration is only noticeable in that elusive place between acceleration and deceleration where these things tend to crop up. Reminds me of a problem I had with a pinion gear bearing once but I hope it's not in the rear end. Is there a center carrier bearing or mount on the driveline on this car? What about u-joints or cv-joints? What's the best we to check the action in the drivetrain to isolate the problem?
Thanks for any suggestions,

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