I'd recommend 4WD, like a car you will spend most of your time in 2WD but you'll be glad you have the option when you need it, up in the kimberley you will encounter rocks ,rocks and more rocks of all sizes, sandy creek beds, serious creek banks and even big spinifex clumps can be hard going.
Take lots of tyre plugs and a compressor or fit 8 ply tyres to reduce punctures and still take the plugs and compressor.
Have fun, it's awsome country.
cheers gaff