1999 Monte Struts--How Difficult?

I have a 99 Monte LS with about 75k miles. I just got new tires put on. Starting at about 67 MPH, the car starts shaking. At about 80 MPH it feels like I'm sitting in a paint shaker. Since the struts are original, I fear that they may be the problem (if you have other ideas, let me know).

The car first started shaking about 1-1/2 years ago right after I got the brakes worked on at the dealer (at about 57k miles). I just assumed that they knocked a tire out of balance when they dropped the wheels. I was going to get new tires eventually, so I didn't go back to complain.

Is there anyone around here who has done Monte Struts before? How difficult are they to do? Is it something that someone who's never done them before could do?

Thanks for your input.

I don't think your problem is the struts. If you got new tires and the problem is still the same, something else is making the car shake (obviously), but I don't see how it could be the struts. Are you sure you have no bent rims? If not, you could possibly have a bad drive axle, or other transmission related component, or bad engine/transmission mounts possibly.