2002 Chevy Silverado

I am getting a vibration between 55 & 65. I lowered the rear about 1.5 and had no problems for years. It started gradually and I had front end work done because that is what the shop said was the problem. It did not sove the issue. I was told to put angle shims to correct the angle. I called Stylin and they said that just lowering that much would not cause the problem. I had a 99 new body and lowered it the same and never had a sinle issue. Could you give me a couple of suggestions?