Bad window seal?
Should be in the compartment by the left rear wheel inside the vehicle.
Make sure it is oil and not brake fluid.
Overheating can damage any seal.
The same reservoir that serves front wipers serves the rear window. If the rear sprayhead is not working then it may be blocked or the line to the engine compartment may be pinched, or have come disconnected.
The 2011 GMC Yukon has 65.2 in. of rear shoulder room.
The 2005 GMC Yukon has 65.1 in. of rear shoulder room.
The 2015 GMC Yukon has 60.3 in. of rear hip room.
The 2001 GMC Yukon has 38.6 in. of rear leg room.
The 2012 GMC Yukon has 60.6 in. of rear hip room.
The 2012 GMC Yukon has 39.0 in. of rear leg room.
The 2002 GMC Yukon has 39.4 in. of rear head room.
The 2008 GMC Yukon has 39.2 in. of rear head room.