I just got my rack and pinion put on since I had my steering wheel has been shaking/vibrating.
My car has 140000 miles.
My car has an automatic transmission.
If there was no vibration prior to this work, I would first check to make sure that the rack is properly torqued to the body and also check the tie rod ends. If any of these connections or fasteners are loose or missing, that can create the problem you are describing.
Also, check to be sure the front end was aligned properly after the rack was installed. If all is well there, you likely have a tire/wheel imbalance and/or a defective tire such as one with a belt separation. Obviously, you should return the vehicle to the original shop to have them check all this out. If you would like a second opinion, you can enlist the help of a certified technician, like one from YourMechanic, to inspect the steering and suspension and advise you on any needed fix.
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