The tailgate on my 1200 ute is quite loose when locked in. I cant personally see how it can be adjusted so the that it still sits flush with the rest of the tub. If anyone has some tricks from experience could you send them my way? Cheers!

Cut up some 8-10mm rubber hose. Split it down one side. Put it around the tube piece on the handle side. The rubber will then squeeze against the hook on the gate.
Works for me. No more annoying vibrations

The ute handles should be almost pulling the sides of the ute tray in when clamped shut. I think there is a small amount of adjustment with the handles if you loosen off the two bolts behind them (you will need to pull off the inner panel to get to the inner bolts) as they have an elongated hole they bolt through. The other thing that might be causing it is if the hooks that the handles latch to have been worn away, in which case I have heard of people putting some weld in them and filing it back.

Otherwise...simplest solution maybe just following the previous solution by uteboy1200 with the rubber tube.

I've adjusted them fully via the mounting holes on the tail gate. Helped a little but not much.I'll give your idea a go ute boy. Cheers for the feedback guys!