Have you changed the Trans Fluid previously - Normally it should be changed about every 30K. Transfer case and Diffs should be changed out also at 100K - Use synthetic GM products when you can. If it's never been changed, I would hesitant to chance it at this point.
Wow, short post, but yet you managed so much bad information.
The GM owners manual for the OP's truck states trans fluid should be changed every 100k with "normal service," and every 50k for "severe service." Not 30k.
Same owners manual states every 50k for the transfer case. Not 100k.
Owners manual also states synthetic for the rear axle, not "when you can." And it actually does not state any recommended change interval.
And lastly, FFS, please stop spreading the "it's never been changed, so don't bother doing it" myth. If you accept that statement, you're basically resigning yourself to a transmission failure. "The fluid is completely shot, has no lubricity and the additives are gone, but gosh, let's keep running it!" That's a guarantee of the transmission failing.
Even if a transmission has 200k and it's never been serviced, change the fluid. One of two things will happen:
1) Fluid is changed, everything works. You're ahead of the game.
2) Fluid is changed - transmission fails. Unlikely, but this was going to happen FOR SURE if you didn't change the fluid.
I beat the crap out of my truck, so I do the trans (flush), transfer and axles every 50k. Since the power steering also powers the brake system, I've also had that replaced twice, at irregular intervals. Truck is at 195k and is still all-original. I fully expect it to go at least 300k.
One more thing - don't think about coolant changes in the context of "will it still protect against cold temperatures?" Think of it in the context of "does it still provide lubricity for the water pump?" Letting your coolant go too long will lead to premature water pump failure.