First time I saw these: http://www.fieldmasterwheelchairs.co.uk/Outdoor.html
4 motors, front wheel steering like a scooter...
does seem to work in the mud: http://www.youtube.com/watch?v=1xmSn1LJ0kw&feature=endscreen&NR=1
And they have a hire service which is kinda cool.
You can hire a 4 x 4 for a holiday in a forest and return it because it's too bulky to use at home.