When I do my exhaust on the 2 post lift and have the rear at full droop, I move the X right up to the driveshaft. Usually not actually touching but it really doesn't matter much if it does because in real world conditions with the car on the ground you'll have clearance. The only time the rear would be at full droop like that is if you're Bo Duking rail road tracks, lol. So you don't really need any "clearance" per say when the rear is at full droop on the lift. Typically the X's are installed fairly close to the yoke area anyway, or within 12" of the front of the driveshaft where driveshaft movement is very minimal.
I have a a 4" aluminum driveshaft in one car (Abody) with a 3" exhaust and X system and it tucks up fairly well and has never touched the driveshaft during any driving conditions.
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2019 Pontiac Heaven class winner
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