After rebuilding my 2F in my 1985 FJ60, I had the pulley walk off the shaft after about 1500 trouble-free miles. I had used my impact wrench to install the nut, and during the hours of recovering the vehicle, I had plenty of time to not remember torquing the nut with a torque wrench, unfortunately.
The nut is somewhere along the highway, and the key was pulverized and ground into silvery goo. It wasn't fun, but it could have been way worse. I got a used replacement nut and key from CruiserParts, and thanks to this thread and other related threads, I chose the following course of action, which I'll characterize as FSM plus harmless voodoo:
1. Clean threads thoroughly, and oil lightly, also remember to smear oil on the underside of the nut flange that contacts the pulley.
2. Apply some blue threadlocker to the threads (voodoo #1).
3. Use web strapping around pulley, hooked to frame, to hold pulley in place while tightening nut to 144 ft-lb with an actual torque wrench.
4. Ponder the lack of a locking tab. Think a lot about how much it sucked to lose the first nut. Fabricate a locking plate from scrap, with 2 M10-1.5 bolts fitting 4-5 threads into the tapped puller holes in the pulley (voodoo #2).
Hey, I know the FSM is scripture, but a bit of voodoo can't hurt. Leaving this plus a few pics for others, since I benefit from this site all the time.