I'm getting ready soon to start fabing a new rear bumper that will mimick this in basic design.
http://www.reganracing.com/bronco/rearbumper/bumper_105s.jpg

What would be the optimum thickness for the metal? 1/4 would be really strong but I don't want to weight the thing down, especially since I'm going to be making one for up front later on too. I'm kinda leening towards 3/16's or 1/8 (would 1/8 distort if I had to be pulled or winched out?). What's your imput?

3/16 would prob be the best. imo

my huge front bumper is made of 1/4" and it'll take any abuse thrown at it... 3/16" would probably be best for strength/weight... my front weighs about 180lbs without the winch

3/16. It's even popular with the aftermarket guys who have a lot to lose if their bumper fails.