2000 5.9L Overheating

My buddy owns a 2000 Durango with the 5.9L V8. He said it was overheating and the heater never gets warm at all. I went and replaced the thermostat. The Durango is still overheating. My next bet is a weak water pump since the heater isn't blowing cold. Any suggestions would be great. Thanks!

Just checking but the coolant is full ? Normally if the heater core is not heating up I have found that the vehicle is either a) low on coolant, b) heater core plugged, or selector valve malfunction. Does the upper radiator hose get hot ? If not you might not have all of the air out of the system after replacing the thermostat or you may have gotten a bad thermostat. Seen it happen.

Just checking but the coolant is full ? Normally if the heater core is not heating up I have found that the vehicle is either a) low on coolant, b) heater core plugged, or selector valve malfunction. Does the upper radiator hose get hot ? If not you might not have all of the air out of the system after replacing the thermostat or you may have gotten a bad thermostat. Seen it happen.

Well I went to check it again today. I took it out and beat on it a little to get it hot. It took awhile for the needle to even start moving. The heater still blows cold and the hose for the thermostat is pretty cold indicating that it hasn't opened. However, the water pump has started to make a pretty good whining noise. This leads me to believe that the bearing inside is going bad and is not pumping the anti-freeze through the system. I will replace it after the holiday and see what happens. Hopefully this solves the problem. Thanks for your help!