So does this keep both axles locked in all the time? I thought about one like this for my rear in the 2wd
It is not a locker. I chose it for the front because it is: automatic, silent, maintenance free (no fluid additive, no rebuilding clutch packs), easier on my axle shafts, and allows me to turn as sharp as a long bed truck can.
From Eaton:
The Truetrac is a helical gear limited slip differential, not a “locker”. The Truetrac operates by transferring power from the spinning wheel to the wheel with the most traction. If one tire breaks traction, the amount of rotation (or spin) is controlled by transferring the lost traction to the higher traction wheel. The Detroit Truetrac operates on the principle of "torque biasing”: the typical Truetrac aftermarket differential will consistently deliver approximately three-fold the torque of the slipping wheel to the gripping wheel.
So it can transfer over 3 times the torque to the other side. But 0x3=0 So one wheel needs to at least have some traction. I've been able to keep up with guys with front and rear detroit lockers/ARB air lockers. I have the ORD front springs so I usually don't have a tire up in the air.
I liked it so much a friend of mine put one in the rear of his Silverado as he would get stuck hauling his snowmobile up to the trailhead. He loves it also. Excellent street manners.