While trying to replace my transmission mount I noticed that, first, I couldn't get a wrench on the top nut to get it off and, second, that there is a harmonic balancer-looking thing on my drive shaft that is bolted on to the flex front flex disc.

Mine has a shiny metallic outer ring and black rubber, like a engine or tranny mount, connecting the ring to the drive shaft.

The rubber on mine has come apart in two places but the ring is still attached.

What is this thing and should I replace it?

Also, how do you get that top nut off the transmission mount?

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1993 W124 300D
-297K on the clock as I type this.