[MENTION=102606]infamousRNR[/MENTION] and I just installed a 3rd member with an ARB locker from East Coast Gear Supply for [MENTION=142071]sunsetrunner[/MENTION] and we filled it up with gear oil within 1 hour of installing it and have no leaks. I put a layer of orange FIPG on the axle housing as well as the 3rd member to make sure it made a good seal. When I torqued the nuts to spec, some FIPG squeezed out around the full circumference. The amount I used might have been a little excessive but I'd rather have too much than not enough. When Fred (aka sunsetrunner) was filling the rear diff, he posed the question of what the curing time was but I figured this FIPG was similar to the FIPG for the engine and you can put whatever you were working on back into service immediately without any issues.
We also pulled his front diff to swap it with an ARB locker which was much harder and involved lots of cursing because it's hard to get out and back in. You also have to swap over parts from the original diff like the diff tube and ADD actuator and vacuum and breather lines not to mention and intermediate shaft that was a pain in the ass to pop out.
So, I could be wrong but I think you can fill it up immediately after you get the axles back in and bolted up. But, I hope somebody will chime in with a definitive answer because I'm just guessing based off the experience we had with Fred's rig.
Edit: [MENTION=57546]4-Ripcord[/MENTION] responded while I was typing. So, I guess by the time we got the axles in, the brake lines connected, and the parking brakes connected back up, at lease a 1/2 hour passed and that's why we were good to go.
BTW, a video of the front diff swap is coming soon to a theatre near you.