First thing i'd do is check & fill coolant as needed, then monitor level for possible leak. Make sure you run the engine when you add coolant, and make sure heater is on. Let it run for quite a while with cap off, to disperse air in system. When coolant level gets low, engine needs all it can get which will cause it to sort of by-pass the heater, until level gets back up where it should be. Low level explains reason for engine heating up as well as no heat inside. Hopefully the reason for the low level isn't anything too bad, or expensive, but you'll know soon enough if you monitor the coolant level faithfully, or if not sure, have your system pressure tested to avoid major problems in future, if nothing major yet.

Also check your thermostat they tend to get stuck in these trucks