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Your adjusment screw is tweaking the depth of your pitman shaft... by rights your pitman shaft won't drop out of your steering box with the adjustment nut completely backed off... I doubt you'll find a o-ring between the adjustment hold-down nut an the cover, you may find a copper crush washer similar to a banjo fitting. May dad's 06 just started leaking... he'll be taking it to the dealer next Monday to have it checked out, I have the feeling his is leaking at the pitman adjusting plate seal... kind of a big job to replace that seal. Just my $0.02 hope this helps.
There is probably a seal in there not an o-ring. From my experience with the gear boxes on my 01, once it starts leaking its time to start thinking abouta new one.