I'm going to guess that your t-case is a NP249 ( thats what mine had too). I replaced it with an NP231 (2wd or 4 hi/lo). There are some write ups online about the swap. If I remember there is an issue about the input shaft length or number of splines so do some research about that.
JB conversions has them for 975.00 all tricked out and ready to bolt in
http://www.jbconversions.com/pages/
The noise hearing is probably the viscous coupler. If you don't really need full time 4x4 I think you'd be better of swapping it out for the 231, and installing a slip yolke eliminator while your at it. (SYE for short).
Check out http://www.mallcrawlin.com/forum/index.php
there will be lots of info for you there if you haven't already.
Here is a thread about the swap itself
http://www.mallcrawlin.com/forum/showthread.php?t=2781
Good luck
