Before you do anything, with the engine at full temp, touch both heater core hoses coming from the firewall. If the heater core is fine then they both will be hot. If only ones hot then the core is plugged. If they're both hot but you're still not getting heat then check the blend door for the heater under the dash. With the key in the ON position (truck not running), when you change the heater temp knob from warm to cool, listen down by the passenger floor and you should lightly hear the blend doors electric motor running as it swings the door one way or another.
'01 Dodge Ram 2500 4x4 5spd Sport, EZ, Straight piped, aFe intake, Rancho 9000x's, 285's, Pac Brake, 2 lo kit, Dakota FP and Isspro boost/EGT gauges on the pillar, Raptor with 1/2" big line kit.