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Thanks for the replies. I had indeed immobilized the drive shaft yoke at its input to the diff, so that should eliminate driveshaft u-joints. And yes, I had read JM's definitive post several times. But I do suspect that this is one of those 5% of situations where the diff is indeed the problem. BTW, I did perform a similar 'wheel-rotation-slack' test on another car I happen to have currently on stands, and it had less that 1/4 of the slop I saw on the Z, and the other car has an LSD which I suspect adds even more slop. So I assume that I've got excessive backlash either between ring and pinion, or side-gear to pinion-mate. Or the pinion shaft is wallowing around in the differential case. I'm sure I'll need more help once I get in there.

On the stock tank? Yes. I haven't started the poly tank install yet. I'm convinced Nissan added the vent on the left side of the tank to deal with the situation. There is no other reason for it to be there. You have to be in just the right climate and usage for it to be a problem. Cool morning fill up, and then parked in the sun all day. There is a real potential to form a vapor bubble there and it expands at a greater rate than liquid. Since I have that plugged the vapors path to release is to force all of the gas that is in the upper section of the tank out if it can. A proper check valve system in the vent line to atmosphere would probably solve it but I'm not crazy about having a pressurized tank. I've just finished testing some polyurethane tubing submerged in gasoline for a few weeks with no sign of degradation. I have a plan to use that as an in-tank vent line. I'll post pictures when I finish it. I also ordered a Tanks Inc. 6" backing ring and I'm going to machine my own custom top plate to provide the inlets and outlets I want.

Before removing the diff, I would check the u-joints on your driveshaft as well.