tailgate
Wow, a sandstorm that does that. It's amazing when you hear these stories. We get nothing like that over here in Blighty!!
Anyway, tailgate set is the overall fitment of the tailgate. If you notice, the tailgate is bolted to the hinge, then thats fixed to something else and so on.
The fitment of all the related components is crucial.
A couple of mm awry somewhere and you will have a bad fit.
I'm assuming that your tailgate has a very minor "set" problem as your squeek has developed sometime after build and test so it was not evident at the factory.
Of course it could be latch,seal,wireloom boot,damper,damper fixing points,hinges,hinge fix points,glass fix or finishers,inner finisher (4 of them - lower upper left and right), grab handle and its components, wiper and components or any clash/combination of those!.
However, feel free to ask your dealer to check the panel set of the tailgate by all means. He will anyway, but at least if youv'e asked he knows to make sure he does a good job!!
I hope you recieve the service and result you should expect.
All the best.