I was just wandering when a good time was to switch the factory fluids out of the diffs and the trans. and to flush the radiator. I've heard that some people do it at anywhere between 5,000-50,000 miles I was thinking about doing it at 12,000 miles. I am just looking for when other people do it that's all. I will probably do it at 12,000 miles anyways. Any input is greatly appreciated.

If you're doing any towing, I'd change the rear diff after about the first 500 miles of towing. Allison will tell you to change the spin-on tranny filter no later than 5k the first time under any driving conditions. The front diff is kind of up to you, but I'm going to change mine for the second time @ 45k. It may look kind of bad the first time, some strange stuff they put in new. I actually went to synthetic in the front, 75-90 same as the back. Went to BG synthetic in the tranny and transfer case too. I think 12k is much sooner than necessary on the coolant flush. That's my 2 cents worth. You'll get answers on here from one extreme to the other. Intervals are dictated a lot by how you use your vehicle.

As mentioned, change the diffs and engine oil at 500-1000mi. I'd change the tranny filter at this time too. While you're under there, might as well change your transfer case fluid in 4x4. My 2 cents.:ro)

I did everything but the coolant, brakes, power steering, and Trans in the first 2500.

Run the engine with Dino juice for atleast 10K. That will help to seat the rings, then start running synthetic in the engine. You wont regret spending the extra for syn. Its worth every penny to protect the 40+K investment.

I would not worry about about the coolant until 3 or 4 years. Get rid of drivetrain oils asap. Change oil & filter every 4000 to 6000 miles, and tranny oem filter same as oil. Fill Ally filter before install, and clean magnet on the filter, & reinstall.