I put 18" wheels on my car a week ago. I finally lowered it yesterday. I also put half shaft spacers in. my car has 28k miles. I noticed when im about 70-80mph and full throttle (doesnt matter what gear) i feel vibrating from the rear end. I would really doubt that my axles are bad as the car is like brand new, and was all stock when i got it. Could my balance be off, or do i need hub centric rings for the wheels?

 

Hub centric rings are useless and only serve to make mounting easier. If it's only under acceleration you just described axle cup pitting to the t. If that's the case, it's strange your spacers didn't fix it.

My guess is you are in need of an alignment from the lowering (regardless, a good idea anyways).