Blade is probably bent, crankshaft may be bent. Replace it with a Toro or Lawnboy. Good luck.

A cheap and easy way to quickly tell if it is just a bent blade is to replace it and give it a try. If the shaking is gone then you paid less than the cost of a repair shop diagnosis. If it doesn't then you have something more serious that could cost real money to fix. If it is a bent crank there are some shops that will try to bend it back into shape. I'm not a believer in that practice, but some have done it successfully. If this mower has a BBC that unit could also be damaged.

If the problem is serious you will need to balance the cost to repair against the age and condition of the unit and the price of a new one. I'm thinking that if it is 10 years old and the repair is over $250 then it may be time to get a new one.

I re-read your message. Given that it shakes with the BBC disengaged I'd say the damage is serious. Don't bother with a blade replacement but get an estimate from a good shop.

Thank you for the replies. I tried to describe the problem to the lawn mower repair shop.

He seemed pretty sure it was the crankshaft, which is what I had (and you all too!) thought. His guestimate to fix it was $150.

Thanks again!

If it was the shaft wouldn't it shake worse with the blades engaged cuz that's when the shaft is spinning if the blades are engaged the shaft not spinning see what I'm getting at