While it's already been touched upon, high mobility that can't be blown out from beneath something punchy would be the most ideal way to deal with a mobile shooty army.
While trucks are good times, they're really easy to de-mobilize, forcing small units to footslog, which is a very bad thing.
I don't know if the Tau have anything that can ignore cover, but if not, a couple small squads of Gretchin / Orks behind an ADL, [Go to ground... I mean, why not?
] Take a Quad... that makes Gretchin a bit more favourable, and maybe some Killa Kans with Rokkits or KMB's, depending on preference. Maybe some Big Gunz... A couple of troop units that can go to ground behind an ADL will lose a bit of shooty power, but gain a ton of survivability, and if your opponent is wasting shots on them... then that's all to the good.
From there, I'd probably look at a couple of large mobz in Battlewagonz should keep them alive to score objectives
Last, I'd max out on Warbikes, Deff Koptas, and maybe even Nob Bikerz.
If you take Wazdakka, those bikes could be troops, and you could take more Bikes / Koptaz on top of those as FA still.
This might be one of those situations where 'Ard Boyz could be useful, since Markerlights don't do anything to their save.
Coming from the experience of having played sit'n'shoot Guard, one of the scariest things for me to face were units that couldn't have their mobility crippled, like bikes, jump packs, and hormagaunts.