Thanks, Tim. I tried microwaving the gasket for the back window this morning as there is no sun to be found down here either today (or the rest of the week, according to the forecast). Go figure.
Just gave it another try with no success. I've done the string trick before with great success but this gasket is just fighting me the whole way. It didn't help that the wife decided this would be a good time for me to watch the 1-1/2 year old and 3-1/2 year old kids in the garage. I spent as much time yelling at them as I did yelling at the gasket.
The directions for the rear window in the Chevy manual tell me to put the gasket on the truck and then put the window into the gasket. Tried that too with no luck.
I suppose I'll switch back to the front window. I think I'm going to install the gasket with the groove and just pretend it's the right gasket. I have to get SOMETHING done this weekend.