Do you mean that the entire transfer case is slipping into low range? Like the low range indicator light is illuminated in the dash lights? If so i'm afraid there is something horribly wrong going on. As far as I am aware I don't think the transfer case can shift while its in drive. Let alone without breaking something trying to do the shift at such a high speed. Im not even really sure what could cause this. Please if you can elaborate on what you mean by low gear because I don't think your rover is randomly jumping into low range. If it is I would let the dealer tackle it, theres some sort of strange electric gremlin going on.