Sounds like you may have lost a wheel weight on one of your rear slicks. If you balance them. It’s possible one of them came out of balance whether you balanced them when you mounted them or not. I have had to re balance mine after some usage in the past. I use a harbor freight bubble balancer, sticky wheel weights, and I duct tape all of my weights to the rim.
Not saying this is the problem, but it’s a possibility.
RJ