I would say what Flatie mentioned.....if by any chance it had been wrecked, I had a buddy who bought a salvaged Rubicon and had the same issue not to mention having to replace cracked control arm mounts.
Given you purchased a used one though, it could be tire balance and it could be the ball joints as Flatie addressed. Also check out your drag link by having someone slowing turning your steering wheel left to right while you watch for any movement at the joint ends. If the vibration is actually a light "shaking", check your track bars and torque along with control arms.
But whatever you do, look into the vibration first. I have known guys who went out and bought beefier steering stabilizers and "dual stabilizer" kits to settle the vibrations/steering and all you are doing is masking the issue.