I tend to use aperture priority mode most of the time when I am shooting wildlife. There is usually significant movement of my subjects in relation to my light source (the sun), and I find that I can change my exposure compensation much faster. You can read my article on depth of field from an earlier post on this site.
Lion cubs, Serengeti National Park, Tanzania, January 2004
Canon 10D, 300mm f/2.8 L IS + 1.4x, 1/250 @ f/4.5, ISO 400
Camera and Lens stabilization
Almost all of my wildlife photography in Africa is taken from a bean bag. There are so few opportunities to get out and use a tripod, and any other means of supporting your cameras and lenses just aren't as effective. However, east African safaris typically have enclosed Land Rover/Land Cruiser vehicles with pop-tops. These are very different from southern African vehicles with open sides. I will be in Botswana in a few months, and will figure out what works best during the safari. I will be prepared to use a variety of solutions, and will settle on one approach within the first few days.
Canon 20D, 100-400mm, 1/25 @ f/22, ISO 100
Canon 1DMkII, 100-400mm, 1/1000 @ f/7.1, ISO 250