The short story is that early during WW2 the US entertained special orders from allied nations like Great Britain, Canada, The Netherlands, France - anyone with cash, really.
Very soon however, when they got into the war also and the US production capacity had to be fully deployed, standardisation was the order of the day.
So you either got US Ordnance spec. Willys MB's or Ford GPW's, or you got nothing. As per Henry Ford: you can have them any colour you want, as long as it is Olive Drab
Also, jeeps from the various contracts were exchanged between the British and Canadians to let supply meet demand.
Read the thread
Canadian contract jeeps for some more background info.