Flushing the heater core in a 17 year old truck with the original heater core is asking for a leak that will fog up the windows and make the cab smell like coolant. Do not flush the heater core unless you are willing to replace it. Instead, take a measure of the upper radiator hose, thats the best case for heater temp, comparing that to the heater hoses will give you a rough indication to the amount of flow through the heater core. Another option is to separate the heater hose on the passenger side of the engine and see how much flow there is, then separate the passenger side and do the same, the flow should be very close to each other.
Coolant color and composition when you last changed it will also give you a good indication as to heater core health. And if you had to replace the radiator because it was leaking or clogged, there is a pretty good chance the heater core is in the same condition, as its just a smaller radiator.