Wondering how difficult it would be to remove the tub from a AU-BA ute and put a tray on it?
The reason I am asking is that I have seen a 03 BA ute that I am considering buying but the tub has a heap of dents in it and I like the idea of a custom made tray.

The tub swap is easy.
Its eight bolts. Myself , Dave , Ross and Wade changed my one from a tray to a tub in 3 and a bit hours.
The tub is fairly heavy and awkward to lift so a few blokes are needed.
There are a couple of items that are a necessity to do the job. We had them and couldnt of doen the job without them.
1. 3 Pizzas and a couple of Garlic breads. Brand is not important. Due to location and time of night we settled for Dominoes as they delivered and it was pissing down rain.
2. A 10 block of Jack Daniels cans. Vitally important piece of equipment. The job wasnt even started until they were opened and quality control tested.
You will also need a couple of spanners.

How about the tail light wiring? Is it just a plug and play affair if I can get the factory tray back tail lights?

The wiring harness just plugs in above the right hand rear wheel.
If you get the factory lights and harness it will be plug and play but you could use the tubs harness and just hook it up to aftermarket lights.
No harder then wiring a trailer up.

Cheers for the info...Sounds pretty straight forward. Do you have any pics from when you and your mates did the conversion?

This is the kind of tray I'd like to make...

Nah no pictures.
Trust me it was really really basic.