is it a one piece prop or a two?
is it an 8 wheel tipper.
have you had all wheels checked for balance.
have all u js been changed or checked.
are the props or prop in spline align.
is there any play in diff pinion bearings.
check all wheel bearings.
check tracking on all front wheels.
check that all axles are secure in centre pins.
check all tyres or change 1 axle at a time and road test.
check that trailing axle bushes ,mtgs etc are secure and without play or flopping sideways etc.
check all prop centre bearings.
check g/box bearings
check flywheel & clutch.
check cooling fan has all blades on.
if these are all o.k. then time for a cup of tea and some serious head scratching.best of luck mate and remember it is usually the simplest and cheapest thing wrong and to fix. regards,jack preston.
Jack, that’s why you start with the cheapest and easiest things first when troubleshooting a not so obvious problem.
Your answers pretty much cover anything that vibrates, the cuppa and head scratch is probably the most likely 
2nd steer axle spring pins & bushes , give them a go ( not expensive & a common failure)