The simplest and least expensive thing to look at first is a broken or loose
motor or transmission mount. Going from Park to Reverse doesn't involve the
transfer case at all. If your truck goes from 2wd to 4wd and back with no
problems then I think we can rule out your transfer case.
The next thing to consider is a tranny problem, and I'm not informed enough
to tell you what might be wrong at that point.
Cheers - Jonathan
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