ljuljati

1. (+ acc.) to rock; to swing
   Majka ljulja bebu da zaspi. — The mother is rocking the baby to sleep.
   Vetar je ljuljao grane drveća. — The wind was swaying the tree branches.
2. (ljuljati se) to swing; to rock; to sway
   Deca se ljuljaju u parku. — The kids are swinging in the park.
   Brod se ljuljao na talasima. — The ship was rocking on the waves.
3. (ljuljati se, of an object) to wobble; to be loose
   Ljulja mi se zub. — I have a loose tooth.
   Ova stolica se ljulja. — This chair wobbles.

The perfective for setting something rocking is zaljuljati; for rocking to sleep, uljuljati.