Bearing wear in the t-case will not cause a loose nut. The nut tightens down on the race, it doesn't preload anything like a tapered bearing (pinion for example). T-case bearings very, VERY rarely go bad, and it is normal to have slight play in the flange. You can look up the spec in a factory manual if you're worried about it.

'97 4Runner, '06 F350, '86 4Runner, '05 WR450
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