Steering wheel vibrating and shaking, usually at high speeds?

The first thing I noticed was when I turned my steering wheel soon after starting it, and at low speeds, there was quite a loud grinding sound when I turned the wheel left or right. That has been going on for 3 weeks or so but did not seem to alarming. Then, 1 week ago, the steering wheel started shaking and vibrating quite violently on the highway. That is scary. It doesn't do it the whole time, but off and on at higher speeds. It happened at lower speeds a few times after driving on a bumpy road, but it didn't last long.

I took it to a mechanic, and they did a complete vehicle inspection. They found a nail in one tire, which they replaced, and then they rotated all four tires. They thought that it would fix the vibrating steering wheel problem.

They thought the grinding when turning sound was probably the struts, but that would be a $1000 fix, and was not a safety issue.

Next time on the highway, I had the same vibrating steering wheel problem.

My car is getting pretty old and rusty, so I'm not sure how much money I want to put into something that is not a safety issue.

What are your thoughts on this problem? And is it a safety issue? Or just annoying.

Thanks,

Amy