One of these would be awesome!!!
http://en.wikipedia.org/wiki/Mine_flail
But lets say you cant get that military grade option.....why not go scavenge an automated car wash place that has the spinning scrubbers, Tilt it side ways and remove the rubber and mount hardened chain with some larger steel balls or something.
Then you can wade through the walkers pretty well. More then likely not having to get out of the vehicle to kill them later. Put some "gun ports" on the sides so you can push a stake or pointy object through and kill the ones getting up on your vehicle.