If it's off a wrecked 4Runner - look for a bent axle housing. And bent axles - although you'd be able to easily swap your axles in. Spin the pinion and see how it feels, should be smooth, no 'bearing' noises. Look for oil leaks around the diff. Look for rust on the housing - if it perforates it's not easy to fix. Wait, Phoenix? lolcry, that's a sad rustbelt fact of life.

It would be nice to test the actuator - since those can fail, but that's not really easy to do unless the truck it's on is powered up. And usually, worst case is you'd just need to take it apart, clean it up well, and reassemble.

The front diff doesn't really have a 3rd member - it's a 'clamshell' case - two-piece case bolted around the differential/ring/pinion. And it's a real PITA to remove and replace. There's just very little room between it, the crossmember it's draped over, and the engine. It's heavy, awkward, and it's a real puzzle to wiggle it *just* right to get it out. Unbolting the driver side motor mount and tilting the motor over to the passenger side would help. And you have to take both sides of the front suspension apart to remove the CV shafts as well. It's easily 3 times the job swapping the rear axle is.