Yep. I have shipped two roosters before.
They were bantams and about 8 months old though, so not too large/heavy. It will be expensive, especially if the roosters in question are large breeds. What kind of chickens are they?
First you need a shipping box that is accepted by the post office. Horizon boxes are good (web site here:
http://www.hm-e.net/) I can ship two bantams in the economy box (
http://www.fortune3.com/Shop@Horizon/USPS_Shippers-Economy_NESTs.html), but if you are talking about 2 full-grown, heavy breed roosters, you're going to need one of the bigger boxes w/a divider. Then you need to talk to your post office. Find out when the best time of day will be to bring them in so that they aren't sitting around on a loading dock for hours on end. Of course, they can only be shipped Express (overnight delivery).
It is also a good idea to prepare the birds ahead of time by getting them used to eating some sort of fruit as a treat. Fruits with a high water content (such as melons, cucumbers are good, too) can be placed in the box with the birds for the trip so that they have something to nibble/a source of liquid. You need to put some sort of absorbent litter in the box as well. Pine shavings are good. I also throw some scratch or crumbles in the box.
But...I would expect that you are going to spend $60+ to ship two birds, on top of what it will cost you to buy the shipping box. My guess is that it won't cost that much in fuel to drive the 100 miles.