My 2012 had the same problem...I have around 7500 miles on it now and it seems to have just went away. I had the cruise control set on 68 mph the other day on a 80 mile trip with no problem.
My friend has a base 11 Laredo with the 16 inch rims. He has no problem with the shaking or trans shifting...So i wonder if its the 20 inch rims are the cause of these problems. I know the overall diameter of all the wheel and tire combos are the same but maybe there is just a slight difference some how.
Back in 2007 I was going to buy a magnum R/T with 20 inch rims on it...Then I drove it on the highway and I thought the car was going to shake apart. I went and looked up the problem and there was a ton of complaints and the problem wasn't getting fix.