hydrolic sensors on 2003 Lexus LX470

When driving at 20 mph feels like i'm running over bumps in the road but it's rythmic (bum...bum...bum). Below or above 20 it's okay. The mechanic says it's the hydrolic sensors going bad? I think it could be unbalanced tires. Any suggestions?

Asked by for the 2003 Lexus LX470