I have an 03' Chevy TrailBlazer EXT with the 5.3L V8 (LM4). This truck mainly sits because I generally opt to drive th 03' Monte Carlo, but that decided to blow the intake manifold apart so that is currently benched and I'm stuck driving the TrailBlazer. I've had this truck for 2 years and the only time I've put coolant in it was when I bought it 2 years ago. When I started driving it again a few weeks ago, the heat wasn't warm enough for me, so I topped the coolant off and the heat is great again. This is, again, the first time I've had to top coolant off in this thing in my 2 years of ownership. I've just been hunting the internet and finding all the stuff about the Castech heads cracking and plus reading the service bulletin about it, it has me somewhat worried. The truck has 185k miles on it and runs strong, but I wanna fix whatever I can before something blows. The oil doesn't seem to have sludge in it. Now, the service bulletin lists several different RPO codes for the 5.3L'* that were affected, but the LM4 isn't one of those listed. Does that mean that the LM4 likely isn't to be affected from the cracking heads? Has anybody had any experience with heads cracking on an LM4? I'm a worry wart so I'm just curious if anybody has seen the LM4 with the same cracking head problem.
The casting number on the heads is 862 if that helps any. I'm just trying to see what some of you guys with more experience have seen with these motors.
Thanks in advance for any advice given and sorry for rambling!

I haven't heard of it. Yours likely isn't prone to cracking heads because of a factory defect at 185,000 miles. I'd check for coolant leaking elsewhere in small amounts.

Has the water pump ever been replaced?

Have you checked both tanks on the radiator (front, back, and sides) for cracks?

Lm4 is not listed on TSB 06-06-01-019B so the TSB does not apply to your engine. Also check the water pump to block gaskets. The Ones with the orange had issues with leaking down the front of the engine and not dripping on the ground. Can also look at the 4 air bleed seals.