Sewie's right.
UCP- Under Carriage Protection
http://hummer.dealerfit.com/store/hu...1309_13045.php
Same GM stuff sold at the dealer, where they charge more money. Easy to put on yourself.
Rock rails
http://www.rocky-road.com/rockrail.html
I have these, the GM factory ones are good too and can be found on the site posted for the UCP. Both of these are important if you plan to run any of the more advanced trails.
A CB radio is also a must, everybody communicates with each other while the group is moving.
Gloves, blankets, water, other stuff too. Tow straps are a good thing to have but there will be plenty of others there. There are also several trucks with winches also, but I don't think anybody needed one last year.
* Edit -Well just the once, but several got pulled that time.