Ok I need some help here I installed a set of 2' leveling spacer and it looks good

. But now between 30-40 mph I have a vibration

. Any ware eles in the speed range its all good. Have any of you guys had this problem and if so how do you fix it because I want to keep the lift? Also when I removed the spacers and put the truck back to stock the problem was gone can someone please help me out. Taco Man.