I have contacted the organizers of Dog in a Ute to see what they had to say on this topic. Here is their answer, which I totally agree with:

A ute is a comercial vehicle that requires only a car licence to drive and it has an open cargo carying space.
The first factory ute was a 1934 ford which was made by a request from a farmers wife to take pigs to market on Friday and go to church on Sunday in the one vehicle.
One could remain in the dark ages and say that a ute must have a full side of tin and a well body just as the original version but it is quite clear that the ute has evolved over time as has most machinery and vehicles in general.

A tray back, styleside, crew cab, space cab, crewman Holden, F100, F250, F350 Chev C10, C30, Dodge style sides (1970 vintage) Suzuki Swift, The Volkswagon crew cab of the 60's, the list goes on.(all these are of 3 ton capacity and less. Over that and you require a different licence)
A ute can have a canopy, hard lid, hurdles etc as these are all accessories but they all have two things in common.....
.......1; a passenger vehicle with an open cargo carrying section.
2; They only require the standard car drivers licence.

The yanks call their utes 'pick up trucks' (or trucks for short) as in the F series and GMC, Chev, Dodge Ram. Just as we do not refere to the ute as the 'utility' all that often.
The ute was an Australian invention, the yanks did not adopt our terminology 'ute'.

'Tray backs' are just a version of the ute such as the'Well body', 'Cab chassis'.
I would not like to be the person that try's to convince the thousands of farmers that the things they drive are not utes.
Deni Ute Muster will have a lot of angry and bemused people not allowed into the muster because thay drive a tray back Cruiser ? (and only have half the number for the record).
The spray contactor that removes his tray and puts a chemical tank in its place....it's still a ute.
The plumber that removes the tray and fits a tradesmans tool box in its place...it's still a ute.
Are some dogs not going to be called dogs because they have a different look or manerism about them ?

If that didn't convince you, nothing will...