The Grateful Dead probably played in Chicago more than they played in the entire state of Wisconsin. I think that it's a far bet that if you took a vote among the members of the dead you would hear a jamin 10-minute "Gooooooo Bears!"
I scoured the eBay listings to find all the Grateful Dead Chicago Bears shirts you could get before the big game. There is a pretty good collection.
Grateful Dead Bears Shirt #1, I like the actual bear head in the Stealie, more info here.
Bears Football, Established 1920 with a dancing Grateful Dead dancing bear, more info here.
Grateful Dead Stealie with the Chicago Bears logo in it, more info here.
Another Grateful Dead / Chicago Bears Established 1920 t-shirt but a different style, more info here.
Grateful Dead Soldier Field 1995 T-Shirt Reproduction, more info here.
I know this isn't a Grateful Dead / Bears shirt but any Bears fan will appreciate it. "After further review, the packers still suck", more info here.