[Cailen]

Once your truck is at operating temperature are both lines leading in and out of the heater core hot? Feel em with your hand. If they're both hot then your heater core is probably heating up.

If they aren't you could try backflushing it. There's a really easy way to do it:
-drain a gallon of coolant
-disconnect both the in and out hoses at the heater core
-attach two new 3/8" hoses, one for drain, and one for fill. the drain hose will have to be 4'ish feet long so you can get it over the side of the vehicle.
-take your garden hose and using the pressure nozzle pump a ton of water through that fill hose.

I did this immediately after a Prestone FLUSH. You may consider doing either the flush or the cleaner. The difference is, basically, the flush you add to distilled water and have run through your truck for about 20 minutes of idling (at about 1500 - 2000 rpm). The cleaner you add to distilled water still, but you drive your truck regularly, and after you've accumulated about 6 hours worth of driving you drain it all out.

I would follow up either by a good reverse flushing as I've described. I REALLY don't like those reverse flush T fittings, but some people do so you may consider going that way too. I and other people have found the described method as being very effective.

You might consider removing your rad and backflushing it too. This is a very easy procedure and is described in the Haynes manual.