1998 Z71 Front Diff Leaking

have a 98 z71. the front diff is leaking between the diff housing and the inboard flange. the entire flange that connects the cv axle to the housing is loose and apears to be leaking. also when i engage my 4wheel drive there is a whine that has never been there before. i have never dealt with this before but i am pretty sure that there is some kind of seal and bearing that is bad, someone please shed some light on this for me... how much will this cost me? I am pretty handy with my truck, but i have never worked on a front or rear diff. help

thanks in advance
DB