rear derailleur jockey wheels

qwerty1982 Posts: 232
edited December 2010 in MTB beginners
Im sure a very n00b Q but the wheels on the rear derailleur do not spin freely with out the chain on. As a result any back peddling can cause the chain to come off.
How do I service this? Are they a component that have to be replcaed or can the bearings be cleaned and re greased?