Woah there Tido, you'll have to provide some more information/photos and do some of your own research before you'll get helpful answers.
I have no idea what transfer case is in your car but I imagine it's a Sierra case, but we can't tell you
which Sierra case because we can't tell anything about your car from your post.
Are you sure you mean you need a longer
axle? Because this is what an axle looks like:

It's inside the axle housing, and you definitely don't need a longer one when you add lift.
Are you sure you don't mean the driveshaft?
Which looks like this:

If that's what you mean, you can buy driveshaft spacers from low range off road up to about 30mm in length, which should be plenty.
Having said that though, I'd be a bit worried about your "massive lift" causing driveshaft, steering, and perhaps even transfer case failures, so, with some more information about the car, we might be able to advise you on how to make it more reliable.
Steve.