i had the same problem......bought tires..i have 06 4.4...old wheels 255/50/19...285/40/19....and got new wheels at..275/45/20....315/35/20.....and notice the same thing...and i thought the same thing what you thought @ricky27.......and finally started doing one step at a time....did the balancing through diff shop...and it fix the issue of shaking.......so far nothing else wrong now..completely straight run...