It could be related to the Hill Decent Control Switch on the gear lever.. look to see if the wires have broken off the plug or the plug has come un-done, you can do this by pulling the gaiter down.
I did have a petrol that had this same problem... but the dishonest seller didnt tell me that the IRD was empty and the props and viscous coupling were removed totally making it 2WD, so you may want to look underneath and check its all there before you go any further! I hope this isnt the case for you though!
But, as 'The Mad Hat Man' said i went and got meself a diesel! Best thing to do!
Cheers, Christian.