You do it with everything in place - including the axle (C-clip and all) - so that's not an issue. Once I put the locker in, the ring gear was so thick that it blocked part of the hole that the crosspin slipped in. What I did was unbolt the ring gear from its carrier while it was still installed in the assembly, then slid the entire ring gear all the way to the other side across the locker. It slid far enough over to clear the crosspin hole completely on the ring gear's backside. I put the pin in, then slid the ring gear back over to its place, and rebolted it on the carrier. No grinding required.
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