1999 Dodge Caravan Shimmy in front end of 1999 Dodge Grand

I have a 1999 Dodge Caravan that gets a shimmy in the front end around 65-70 mph. It gets really bad by 75mph. Sometimes so bad that you need to let up off the gas which makes it worst at first then it will let up. I've also noticed it will do it quicker when going up hill or when under a load. I've changed the tires, had the front end aligned and wheels balanced, but the problem still persists. I did notice that the boots on that bottom side of my stabilizer bars were busted, but when I had the front end checked by a local shop they said the front end was in good shape and that those stabilizer bars would not cause the shimmy. I would really appreciate some guidance and help with this issue