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It is my understanding that we do not have slip yokes stock...

you're confused... because we don't have slip yokes on our tcases (you commonly hear tcases referred to as fixed yoke or slip yoke), but the slip is on the shaft itself. If you have a slip yoke tcase, you have no slip built in on the shaft. As mike pointed out, if you eliminated all slip in the system, you would have a problem.

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A freind I trust sudgested having them put a slip yoke conecte to the transfer case outpiut shaft

this is bad advice. you don't want this type of setup... slip at the tcase. jeepers buy expensive kits to elimate this for the setup you already have. you can't get very much travel out of the slip at the tcase. So when offroading and flexing out, this setup can pull the driveshaft out of the tcase. fixed yoke on the tcase and slip in the shaft is far far superior.

-Rob