Slight vibration at high speed

Have a mazda bt50 5 cylinder which has done 12,000klms it drives great real smooth on a trip down to the daly river on about 10klms of slippery mud it was all over the place and abs going crazy, after fishing for couple of hours returned and when on bitumen at 85 I felt a difference like a slight vibration. Passenger even commented as I stopped and checked the tires as I thought I had picked up a nail or a rock, no steering vibration so Ime thinks its coming from the back end. Put it in to Kelly's in Darwin for its 10klm service ($508) thank you and guess what, they said it was different from one of their vehicles but they could not find anything wrong? Surprise....

Any suggestions, have it booked in at pedders in darwin, but not till next month as I,m down the daly fishing from tweed heads nsw.