Here is something I found the other day when investigating drivetrain solutions. We still might use these, but maybe somebody else can as well.
It's pre-made, relatively cheap, can easily made lighter, and all you have to do is bolt some motors in to get it to work. Maybe it will work for you. I called them and they have a lot on stock (plus some other neat things on the site):
http://www.robotcombat.com/store_tanktreads.html
Only $140 + shipping for the set. They are only 20" long, but if you put them on the front of a bot with casters or the like on the back for stability (or even some kind of suspension), it could be a good combination.
Good luck.