I found this:
The Austin K2 ambulance used was four wheel drive, and had presumably been converted for the movie using a front axle and wheels from a Canadian Military Pattern truck, along with a transfer case and prop drive shafts. The hubs are slightly different to that on a normal K2, and the differential housing of the live axle can be seen on some shots. It was probably changed so it could extricate itself more easily in the shots where it was bogged down. There's seems to have been two Austin K2s used for "Katy", as in some shots, it has the driven front axle, and in others, the original beam axle fitted to these vehicles.