Drone noise around 70 mph

[rdunagan]

I installed a 4" rough country lift about 2 or 3 months ago. didnt do the shim kit till last month and i just replaced all my u joints and carrier bearing and this "drone" started about a month ago. it rides fine with a slight vibration (which i think is the tires) but when you roll into the throttle and get to 75-and on it got a bad humming/droning noise. its not my exhaust system i know that nor is it the tires. u joints all look good but i cant figure this out. it only does it around those speeds and gets louder the faster you get. and i might mention you can feel it in the seat and in the petal when it does it. also it will do it every time. i cant tell if the shim kit is causing it or if its the axle or what. its driving me nuts being that i have replaced the u joints and carrier bearing