Oh boy, my manual states that one is only supposed to drive straight forward when the differential is locked. Not only will it tear up your ground, but it could also damage your transmission when you turn with both back tires in lock step. While I was passing time with the mechanic at my Kubota dealer (who was showing me how to field service my unit - oil, lube, etc), he volunteered a whole bunch of interesting anecdotes about tractors brought into the shop for repair (you wouldn't believe some of the stuff folks try with their tractors - could be a new thread - dumb tractor tricks - one of which was some guy's idea of "super 4WD", a locked rear differential and 4WD, until the rear differential crumped). Head for the dealer for service.