05 Z71 transfer case clunk

[bmchevy1979]

Well I recently purchased a 05 Chevy Silverado 4x4 and i am getting ready to service the transfer case and axles on it but on the transfer case when you are shifting from high 4wd to low 4wd it makes a clunk that is fairly loud it doesn't do it any other time just when shifting from high to Low and Low to High. 4wd functions perfectly in both its just fairly loud when switching I'd like to know if it's something I should be concerned about or not the truck is pretty much stock and I plan on using it off road quite often mostly at the sand dunes and in the snow it looks to be the original fluid in the transfer case it's fairly dark compaired to the bright blue that the Autotrak ll fluid is so is it possibly that it's just gummed up from not being used very often and the fluid being old