1999 Mercury Marquis Car shakes on highway at 60mph

Yes I'm inquiring about my car's poor handling on the highway. I just had the shocks replaced and the car still is very bumpy. Also, I had the bushings replaced; which consequently, has resulted in a new sound, a squeaky sound now. I took my car back to the mechanic, and he says the shocks are fine, and doesn't hear the sounds that I hear. I'm really about had it with this car. So, do I need to replace my tires(to better complement the new shocks). Or do I need new springs? I'm really uncertain if I should continue to put money into this car or get a new one. Secondly, the check engine light came on-a code came up regarding the cataylic converter, mechanic says it just might be bad gas, nothing to worry about. Any help, would be most appreciated.