My equipment for the Yala  drives consisted of two setups. For smaller birds and distant subjects, I used a Nikon 400mm f/2.8 VR with TC-20E teleconverter (effectively an 800mm f/5.6) mounted on a D300S body. For lower light and closer subjects, I had a Nikon 70-200mm f/2.8 mounted on a D700 body. Both setups worked well, and didn't leave me feeling like I was lacking focal lengths.

A word of advice on photographing in Yala - be ready with a shorter lens/camera combination at all times! The winding roads coupled with the dense scrub means you may come upon a leopard or bear without warning, and only have a few seconds before the animal disappears back into the bush. You often don't have time to meter or fiddle with other camera settings. I set all my cameras to Aperture Priority mode and shoot wide open, with the exposure compensation set to minus 1 stop. This works for the majority of lighting situations except where you'll have any sky in your image, in which case I change exposure compensation to 0 or +1. In my experience, keeping the exposure comp. at -1 allows me to pick up a camera and shoot quickly when I see an animal on the ground, and wherever a bird or animal is outlined against the sky I usually find I have the time to change the exposure compensation. 

I also use manual ISO, and as light changes I make sure I keep changing the ISO every 15 minutes or so to give me acceptable shutter speeds. You can do this by pointing the lens out the window at the ground while the vehicle is traveling and using the shutter button to turn on the meter and read your shutter speed.