Trucks 4 Wheel went out recently and I'm hearing what sounds like a constant chain slipping noise cooming from the transfer case... checked the shift linkage just in case and it shifts fine. A mechanic friend of mine recommended finding a good used Transfer Case & swapping it in place of my original TC and then tear into it on the bench and rebuild it new ??? The advice sounds good but I'm wondering if I need to go through all the extra expense and time swapping out TC's or would it be better to tear into my original TC while its still on the truck and pray its a relatively easy fix?

 

I would say pretty much anything that will cause the noise will require you to remove the t-case. I assume the truck is a DD? If so and you can find another case for a good price it might be worth it. But I would just rebuild the newly purchased one and only swap it one time.
But to save $$ and if you can have the truck down for a few days, just buy a rebuild kit and fix yours.