Donny,
Your vehicle requires the use of Dexcool coolant. GM recommends flushing at 5 years or 100,000 miles which ever occurs first. In reality, flushing at 30,000 miles or 2 years eliminates the sludge problem. Do not, under any circumstances, use anything other than genuine Dexcool coolant or mix any other type of coolant with it, or you will encounter sludge, guaranteed! The heater core and radiator are typically the first things to be affected by the sludge, and are also the most expensive to replace. This link explains:
http://free-auto-repair-advice.blogs...label/Dex-Cool
If your sludge problem goes untreated, the entire cooling system, (radiator, radiator cap, overflow reservoir, heater core, thermostat, coolant passages in the engine, hoses, intake manifold gaskets, water valve, etc, etc), will become contaminated. I service an entire fleet of 4.3's and I have found that flushing at 30,000 miles virtually eliminates problems with the cooling system, (some of the fleet vehicles have over 300,000 miles and have never had a cooling system problem).
If your system has a leak, which it sounds like it does, leaking intake manifold gaskets are a known problem on 4.3L engines. Cooling system neglect is partially responsible. A cooling system pressure test will tell where the leak is coming from. Hope this helps.