1985 W123

I have a bad vibration, almost like a strong hum, over 45MPH. Above 60MPH it gets a bit scary. After trying the more obvious things, I have now removed the driveshaft. I'd like to run the car in gear to see if the hum is there without the driveshaft in place so I know if it could be originating from the transmission. Is it safe to do this? The car is on jack stands in the back and on ramps in the front.

A bit of history:

• bought new tires and had them balanced. Vibration still there
• ran car in gear with back end up. Vibration still there.
• on highway put car in neutral. Vibration still there.

I think this eliminates anything in the engine, front end, or tire-related.

Next planned steps:

• Center bearing carrier is all ripped up so I'll replace that and the bearing.
• getting the driveshaft balanced.
• replacing rear transmission mount
• replacing the flex discs

A couple of years ago the transmission failed so I had a used one swapped in. The car hasn't been driven much since but in the little I drove it at that point I don't recall this hum/vibration. Whatever the case, I'd now like to return this vehicle back to being the comfortable, reliable daily driver in was previously.

Any further thoughts or advice is welcome.

John