99 silverado z71 4x4 5.3L
so as title says i have a Vibration at about 40mph not 55 or 30 only at 40
dont seem to be tire and gone at 50 onlt stays if i stay at 40 then it feels like bid bubble in tire but tire is good or a bent rear end but i know thats not it

 

Can you pin point if its coming from front or back, in steering wheel, feel it in floor?

 

this is doug
feels like its coming from back but cant be 100% untill i have my girl try and me jump around

 

Are all the bearings in the back good? Did you check for bent driveshafts/axles? There can be a problem with the main driveshaft. Sometimes its actually the busing in the tail shaft in the transmission that's the cause. It is better to have the driveshaft sent out to be measured, balanced and replace the u joints.