I would do it around 15K miles and then maybe every 30K. During breakin, a lot of metal shavings build up on the magnetic plug, and if for no other reason, I would pull the plug to clean it off. Also, if you drive through deep water a lot, your front diff. is susceptible to getting water inside, as it has a vent hose under there. I did my gearbox, transfer case, and both diffs around 15K and all of them had plenty of shavings. Others I know who have changed had the same findings.
I can't remember which variation off the top of my head, but the transfer case takes ATF. i used Redline D4 ATF in mine. Gearbox (assuming you have a manual tranny) takes GL4 75W-90 oil, and both diffs. take GL5 80W-90 oil. If you have limited slip in back, make sure you add LSD additive also.
BTW, changing these is easy to do by yourself with your truck on the ground. You just need a 1/2" drive and extension (no socket; just the drive) and a pump of some sort to pump the oil in.
HTH!
------------------
[This message has been edited by ChuckH (edited 02-28-2001).]
_________________________
ChuckH
"Every morning when I wake up I know it's not going to get any better 'til I go back to sleep again!" Al Bundy