Auto Air Conditioning & Heater Repair: No heat, dollar job, engine temp

Question
My 1997 Blazer heater wont work correctly. Engine temp is about 180 deg. When temp.control is turned to hot, no heat will come out of lower vents. If i switch control to cool, cool air will come out lower vent. If i turn heat control back to heat & switch to upper vent, luke warm air will come out upper vent. Water pump never changed, total flush never performed either, but heater core was replaced about 2 years ago. Thanks, Donnie

Answer
Donnie,
It sounds as though it may be time to replace your thermostat. This is usually done when the water pump is changed or around 80000 miles. What I am picturing happening is that as your coolant is heating up, the wax on your thermostat is not reacting as it should (As it heats up it expands, squeezing a tapered rod connected to the "cap". As it squeezes it, it slides and opens) Without it opening the "cap" as far as it should... none or only a trickle of the hot coolant makes itinto the heater core to heat up the cab.
This is the first thing that I would check, I am about 90% certain this is the culprit.
This is a fairly easy replacement that you can do yourself with a couple tools and something to drain the coolant into. Or about an 80-100 dollar job in a garage.
I hope I was able to assist you at least a little bit. If so please remember to rate this answer. If I can do anything further now or in the future, please do not hesitate to ask. Good luck in everything and take care.