I have a 2010 RB E350 I picked up a year ago and I've heard a creaking sound from the rear doors when going slowly across something like the changing pitch of a driveway to a street. This occurs especially when one rear wheel is flexed. It has happened before and after my 4x4 lift and install.
Ditto on the silicone lube. I had a creaky driver's door and used a Q-tip to apply the lubricant to the door's roller. This solved the problem. Ended up lubricating all the doors and have had no issues since.