I know you have the spacers because your tires are 315s, but do you think 285s would require the spacers?

 

Tires could be seperating inside

 

The only way I found this out was the guy at the tire shop just had te same issue with another customer. You can see wear marks on the outside of the spacer. The outer rim isn't centered. There is play between the lugs and the holes in the rim. Having the lip on the outer side of the spacer centers the rim and doesn't allow it to move off center. The guy swapped out the spacers on the other vehicle with spacers that have the lip for the outer rim to sit on and bouncing problem solved.

 

Here is a pic of the spacers with the lip on them

 

Where did ya buy em the bouncing issue sounds exactly like mine I have 3 inch spacers in the rear and it does the bouncing right around 35mph also and has a vibration I cannot find where its coming from makes so much sense now lol

 

Arrowcraft.com is where they are at.

 

Take off your outer dual and look at the marks from the rim on the spacer and you will know what I'm talking about.