Last night I was using my company van, an 08 plate transporter TDI 4motion to its full potential, but when we got back to the public road the rear diff lock failed to disengage. We tried several times starting and stopping, reversing, etc but the light on the dash wouldn’t go out. After 20 minutes of playing the light eventually went off and we headed home... The last thing we want after a long day on site is a 20mph drive home. Anyone got any ideas what might be wrong?