4x4 Issue

[PotentialTahoe]

As some of you all know, I just purchased my first tahoe. 99 4wd.
When I goto put it in 4wd either 4LOW or 4 HIGH I place the trans in neutral and then shift the floor stick into the neutral(orange) zone out of 2Hi. But when I goto put the gear into 4LOW I hear a grinding noise. Also If I try to put it back into 4High, I get a grinding noise. The only way it works if I turn the truck off. Put the floor stick into 4HI, 4Low, or 2Hi then start it say in 4Low then it will be fine, but I'm not sure if is actually in 4wd at that time either because I haven't actually put it to test yet but it drives in that gear of 4Low so I assume it is in activated 4wd. BTW, this is my first owned 4wd truck, so I am new to the 4wd world.

 

[GMCYukalade]

ok, im not sure on the transfer case you have in your truck, but if its like the one in my 99 than there is a fill and drain bolt on the back, and you loosten them w/ a Medium drive socket wrench - you may want to go ahead and change the fluid in the transfer case - now take a note - on my 99 there is a sticker on the transfer that tells you what kind of fluid to use which i think its ATF3... - so there may as well be a sticker on yours telling you what kind of fluid to use

--note--: to fill one of these things your gonna need some type of oil pump, or a funnel w/ a lil flex tube attached to the end of it