[Tiamat-red]

Well, if the old clip is still in there then it's somewhere in the diff. When I took the axle out the first time to replace it the stub shaft came out with it. So I put it in the new one and hammered the pin through outside the car.
And when I replaced it, it slid in all the way(until the axle hub almost touched the diff). But it wasn't secured. I could pull it out far enough so the splines were no longer engaging the differential.
So yesterday I went ahead and, with the "new" clip on, wacked it into the diff. It's now secure. I assume for the first time since I've had it, since the thumping/grinding noise is gone now.
It no longer sounds/feels like a deisel now.
I'm actually thinking that it was coming out far enough under acceleration to unmesh the splines and just start grinding.
I'm still terrified that the old clip is still in there. But it doesn't sound like it.
Both of the axles were rebuilds, so I'm assuming(hoping) that whoever did it the last time forgot the clip.