Hello, looking to replace the gasket between the trans and extension housing on a 2006 4x4 5r55e. Do I have to take the tail, transfer case and extension all off one by one or can I slide the whole assembly off the output shaft? Also, does that nut on the end really need to be replaced or can it be reused?

You must be talking about the transfer case housing connecting to the tranny. Yea, you will have to disconnect both drive lines to replace this gasket. I would just jack the T-case up and move it back a few inches. There shouldn't be any issue with reusing the fasteners, although you can add blue thread lock to it.

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2000 Ford Ranger 4x4(my trusted rig)
2008 Toyota Tacoma 4dr 4x4
2014 Ford Fusion Hybrid
1998 BMW K1200RS - fast BMW with 2 wheels.
2004 HarleyDavidson V-Rod