I agree with other posts that flushing a transmission without first changing the filter is silly. I heard this one guy devised a device suck the atf fluid via the dipstick tube.Supposedly he stuck a tube down and vaccum pumped out the fluid. Sounds plausible, but I am not sure if it would get the atf fluid fron the transfer case. The other concern is getting the tubed jammed in the transmission. This sounds like asking for trouble, but who knows...
The other method would be to disconnect atf return line and use a pump. The outbound ATF tube would be disconnected of course.