Steering wheel vibration

I bought my car brand new about 3.5 years ago and put about 1000 miles on my car during the week with commuting between work, home and school. Since October I've been having the same problem with it. The steering wheel shakes around 55mph-60mph and pulls slightly to the right. When I'm braking it shakes even worse when going past the same mph. Sometimes when I'm driving on the interstate it feels like I'm in a massage chair with all of the shaking. I recently also noticed that my front rims seem to be darker than normal, I'm not sure if its brake dust or just dirt (they've never looked that way before though). My brakes have also started to feel like they're grinding just a bit when I'm almost at a full stop. I'm on the interstate at least two times a day and I don't want to have a brake problem while travelling at that high speed. In October, I made an appointment with my friends mechanic requesting an alignment and told them the issues and over the phone the guy told me that it's a common issue and the brakes need to be replaced. I don't know much about cars but at the time I had around 60000 miles on it so I figured they'd need replacing anyways. I don't remember which set they replaced first, the front or the back, I'll say the front. The shaking was gone for about 3 weeks and returned. So I called them and told them and brought it back in. They drove it and said my back brakes needed to be replaced so I went ahead and it did. Again that lasted a few weeks and again the shaking returned. Because this was around Thanksgiving and then I had finals I had to put it off. But then I had other car issues so I went back to my regular mechanic (he's no longer open on Saturdays, the only day I have to take it in but I didn't trust or like the way the other people just kept having me replace all my brakes to "try to see if the shaking stopped") and he had to replace the knock sensor and the crank position sensor as well as do a 90000 mile service (he suggested to do it since I was at 80000 miles at the time). He checked my brakes and said they were fine and had to do something the front right rotor I think. I don't remember what he said to do to it. It lasted till the past week and now it's returned. I've already put about $1500 into my car with various problems and I'm working 6 days a week and going to school 4 so I can't have the time or money to take my car in so often and then have the same problem recurring. I'd like to have some idea of what could be wrong when I take it in. I do trust the guy I take my car to as I've been going there for years, but it's hard for me to tell him what's wrong when I'm not even sure. Any suggestions you have would be appreciated.
Thank you!