Transfer case is rarely a weak point, which the exception of a known pump rub issue on some cases that has nothing to do with the added stresses of rotational mass or higher HP but rather a design flaw. There are simple fixes to this problem.
As far as transmission, the best single upgrade is a larger transmission cooler as heat is the worst enemy and they tend to slip more causing heat with larger tires.
Next, I would say REGEAR! There is no replacement for mechanical advantage.
The 4l60e is known to have weak points, but there are plenty of less invasive upgrades such as vette servo and a shift kit for a little more reliability.
I have a shift kit for my 4l80e but haven't gotten around to putting it in, about 50k miles now on 37"+ tires and 130k overall with no problems, I do have a 26,000 GVW trans cooler to help out and 5.13s. Other than that there are no simple upgrades to the 4l80e without a full rebuild.
The newer 6l80e and 6l90e's are completely electronic, shift kits and servos will serve no purpose, not much you can do outside of tuning, although they are built pretty strong.
I definitely recommend Cognito Idler and Pitman arm braces.
Heim steering- take it or leave it. It is definitely stronger, but it is more susceptible to accelerated wear due to the environment than the stock tie rod setup. I wouldn't run it in salty, sandy, or muddy environments.
I would definitely look into beefing up the brakes, if nothing else go to stainless lines in the front minimum, there is a night and day difference in pedal feel with large tires.