Chasing vibration...help!!

Tires are like a month old and balanced when mounted
There is some pretty lame tire guys out there. Not to mention new tires get out of balance sometimes easily. It’s a long winded explanation. Another time.
Anyways I had a weird pulsing in and out vibration recently and double checked the balance on the rear tires, 1 wasn’t to bad. The other was way out, and I haven’t felt it since.