Theres a crush sleeve in there. If the pinion nut is loose than your preload is screwed and you'll have to put a new crush sleeve in. They cost like two dollars.
Then you tighten the nut a little and check the bearing preload with an inch pound torque wrench. Keep tightening and crushing the crush sleeve until you get the right preload. I dont remember what the specs are, I'd have to go dig out a manual, theres specs for new and used bearings.
If you're too cheap to go buy yourself a manual let me know, I can rent you one.
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Dan
91 Cherokee, 3" body lift, 2" front lift blocks, Warn XD900i winch, custom narrowed rear seat.