get a prestone flush kit or similar at the autoparts store... 5-10 bucks
Honeywell AFK1T "Prestone" Flush 'N' Fill Kit
the big neck in the kit is to push onto your radiator cap opening and you put the tee in one of your radiator hoses running back to the heater core for a garden hose hook up...

I think that is what you all might have miss is the junk fluid in the heater core... When you flush the truck you need to run the truck engine until hot and keep your cab heat running on high... This makes sure that your thermostat is open and your fluid gets flushed from the block and heater core... after flushing until clear with garden hose water, I usually use light compressed air like 10psi and blow air into the block and heater core (with the truck off of course) to clear the garden hose water out... then refill the radiator and expansion tank with the proper mixture of prestone or other antifreeze... skyfox is right to... you can put unmixed 100% antifreeze in... just do 1/2 of the total system capacity!

this process will ensure you have all the old junk antifreeze out and the proper mixture is back in the block... it has worked perfectly on my vehicles as well as my friends... (25-30 cars and trucks) like i said it works perfectly every time... if you have over 100K you might want to put some alumaseal back in with the antifreeze to plug any small holes or leaks...
Gold Eagle ASBPI12 "Alumaseal" Radiator Sealant