My rear window squeaks like no tomorrow when I drive the truck early in the morning or, oddly enough when the windows are rolled down. I greased and oiled the hinges, to no avail. It seams to be squeaking from the weather stripping or something else rubbing. Any ideas?

Could be the latch and you can tighten that up by wrapping electrical tape around the loop on the lift glass to take out the play.

Also, you might want to look into the interior ventilation check valves on either side of the window at the rear upper corners of the truck.

If you open the lift glass and take a look where the gas window strut is you will see two or three philips plastic screws that you can tighten the rear plates up.

What made me think it is the rear vent valves is you saying that it got worse when you opened the windows.

The window gasket/weather strip is dry
Use a lite coat of vasaline on the entire gasket.
There was a GM TSB about that problem and thats what we did and it fixed everyone.