np 246 autotrac tranfercase clunking

i bought a 2000 chevy 2500 4x4 at an auction
it had two problems it had oil pressure problems and the transfercase was bad

i fixed the oil pressure problem and now im moving on to the transfercase problem

it has the autotrac np 246 transfercase with the automatic 4x4

when i put it in 4x4 it works but it makes a horrible clunking noise

ive heard that these transfercase often have the chain strech and that could be the problem
ive also read that they are supposed to have a special gm only fluid in the transfercase

i havent torn into the transfercase yet so im not sure what kind of fluid it has

but im wondering i someone else that knows more about this transfercase can help

im planning on putting in a new chain and changing the fluid

you guys got any suggestions

thanks  

i have the same problem my chain was bad and some how i filled it with water you do need the gm oil in it though i just got one out of a junk yard but you have to check the number on the case to make sure you get the right one