If it is the flex plate, most likely the front one, replace it soon. It is not hard to do, just takes time getting the exhaust and stuff off. If it lets go completely the drive shaft will flop around doing a lot of damage to the transmission tunnel and anything in it's way. I was only going about 10 mph at the time mine let go. Can only imagine what might happen on the highway if it were to let go there. I have heard of a lot of sheet metal being ripped. I could imagine that it could even hurt a driver or occupant if it happened to hit them! So change it before it goes if that is your problem. My car started vibrating when i let off the accelerator and finally went about 10 miles after it started vibrating.

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Currently
1991 Mazda Miata 174000
1991 Subaru 189,000 Winter beater

got rid of:
1984 VW Vanagon 195,000
1976 350SE Euro 185,000 KM
1973 280SE 4.5 (boy do I regret that)
1980 280SE European W126 4 speed manual (miss it)
1989 BMW 735iL 119000 (good riddance)