Shaking at 1.75 RPM

Hi! I’m in need of some help understanding my car’s problem. My vehicle is shaking when my vehicle hits ~1.75 RPM and the shaking is worse if I’m traveling up hill. Usually, I’m doing between 10-25 mph (city driving). The transmission seems to be shifting just fine and the tires/alignment are in good condition. I took the car in to a repair shop thinking it must need new spark plugs. They came back with a $900.00 repair bill saying it needed a major fuel injection cleaning, plugs and gaskets. I’m a young(ish) woman with a young child without the knowledge to know if I was being taken for a ride or not and had already waited an hour and a half for word back. Paid for the repairs, the work was completed and the car is still shaking. Returned to the shop where they said it was a bad or loose gasket, stated they fixed it and returned it back to me. Still shaking. Is this a fuel injection problem that they didn’t address or likely something else? I’m afraid to walk in there and get another bill if they say it must be something else.