So my Tcase issues are fairly common I guess. Two cracked cases are enough for a lifetime. I could never really figure out why this happened, especially the grenaded one.
I was at the shop checking on the progress of the axle install today and happened to look at the Tcase that just got taken out. Matt, the mechanic showed me something that I'm now pretty sure has been the issue all along.
So on the Tcase drive flange, the nut that holds it on has a wide flange on it. An o-ring gets compressed in a recess and that seals off the fluid.
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But the nuts that were holding the flanges onto the tcase weren't wide enough to stay up out of the recess - instead they fit inside the recess and compressed the oring seal onto the splined shaft AND NEVER SEATED AGAINST ANY METAL. It always had a cushion between fully seating that more than likely allowed the flanges to wobble slightly and created a vibration that caused the cases to fail.
See how it doesn't sit up on the recess, rather it fits inside it.
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So even though I had both shafts balanced (twice) new Ujoints, caster set at 6* etc, etc, the shafts must have been wobbling enough to cause failure. The new Rubi case has nuts with larger shoulders that sit on top of the flange instead of inside it. I really hope this solves the issue. Well, having lockout hubs helps too, but still. Check your nuts!