Ok I tried searching and found nothing so I figured id ask here. I have a 03 GMC Sierra RCSB, 250K miles, bone stock, DJM 4/6 drop, & factory wheels and tires.

I bought the truck with it at stock height and some ugly 20" wheels. All original suspension and I mean all original nothing has been touched. Good driving truck and runs like a champ BUT I have a vibration at about 55-70 MPH. It's there at all speeds but not noticeable really till 55-70 and then after 70 it starts going away.

So first thing I did is change the 20"s and put the stock wheels and tires on there and it didn't change. Next I lowered the truck with a DJM 4/6 drop and only changed the bottom control on the front and a flip kit in the rear with new shocks all around. Still does the same thing. So after I lowered it I took it to be aligned and they said they everything was fine in the alignment.

I feel that it's coming from the rear of the truck becuz the steering wheel doesn't shake at all when it's happening so what do you guys think it might be?

 

Jack the rear end up and put it on jack stands then let the tires role in 1st gear. Check the run out. If it looks like you have a wobble, inspect the axle and the rim and tire.

 

Check your pinion angle, lowering the truck will change that.

 

What is the frequency of the vibration. This will help determine if it's related to the wheels/tires or the driveline.

 

The vibration happens everytime you drive it to the same speeds. I don't think I have a problem with the pinion angle only because it done it at stock height and lowered. Maybe U-joints could be to suspect?

 

By frequency I mean oscillations per second. The driveshaft spins much faster than the wheels, so if its a high frequency vibration then its likely related to the driveshaft. Low speed and I would say its related to the wheels/tires.