You do not "need" the special piston compressor, but its going to make the job a whole hell of alot easier. Otherwise a vice, a big C-clamp, and some vice grips will get the job done. The company that makes the tools for HUMMER is called Kent Moore, and they are expensive to say the least. However, all is not lost for you grasshopper you can buy the tool from Mac tools. I'll post think link at the bottom of this page, 90 bucks and if you ever need to service them in the field you will not have the vice and all that other stuff on hand so I say buy it, not like it can hurt.
As for the Crows foot, they are a specific type of socket. You know the adapters that attach to you ratchet that look like little wrenches? Those are crows feet, they can be bought at any parts strore. Personally I say get a T-handled ratcheting wrench, thats what pretty much every other H1 owner uses to get the job done. They are more effective then a crows foot. Here is a link to all the brake tools you need for the rear brakes, I would reccomend just buying this. Its got everything you need, so you wont need to worry about having the right tools.
http://www.bluehummer.com/brpadretoset.html
The piston compressor:
https://www.mactools.com/portal/site/mactools/menuitem.6c9db894a3743751f735c110f070c3a0/?vgnextoid=024d4f99a75dc010VgnVCM100000104d840aRCR D&vgnextchannel=b8d621e5315fc010VgnVCM100000104d84 0aRCRD&vgnextfmt=default