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.
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