Like I said, an automatic locker really isn't that bad on the street, so I'm not really trying to talk you out of one. But a TrueTrac is nowhere near that bad. It sends something like 3.5x as much torque to the tire on pavement as it does to the tire on wet grass in that situation. The wet grass has to be unbelievably slippery (and I mean that literally) for that to not work. And even if it is that slippery (like if it's on glare ice), as others have said, give it a couple of clicks of the e.brake. That'll make the slipping tire support more torque so the diff will send 3.5x that to the other tire.
For the rock crawling I do, where it's not uncommon to lift a tire in the air, I want the positive action of a locker. But for my hunting rig, that sees muddy and snowy trails as well as mostly good roads, the TrueTrac is definitely the best choice.