1994 Chrysler Le Baron Shaking badly when on interstate &am

I own a 1994 Chrysler LeBaron GTC, 6 cyl. I have had the car about two and a half years and have never had much of a problem with the handling UNTIL NOW! I recently went and had my tires rotated and balanced, and now I am having some major handling issues, at first it was pulling back and forth and was so scarry to take on the interstate that I almost screamed. So I took it back to another tire shop and had them take a once over on it for the 2nd time in 2 days. They did cure it pulling from side to side badly but it still shakes rather badly when I am on the interstate. The 2nd tire repair shop told me there was some mud/dirt on the inside of the front tire/rims and that was causing some of it - got the rims/tires cleaned out and balanced it for me. How I knew it was to good to be true is beyond me - now 75 dollars later I am stuck not being comfortable to take it back on the interstate until the issue is resolved not wanting to cause any other issues. What do you suggest I check into as the cause now? I really thought it needed to be realligned but after mechanic road tested the car they said it was fine just balancing tires. I AM NOT CASH WEALTHY AND JUST NEED TO ADDRESS THE ISSUE HOW DO I NARROW DOWN THE PROBLEM