I have a 2006 xlt that has only had the thermostat replaced (twice in 4 yrs) and the dash relay switch for the heat/ac replaced (once). I'm about to roll 81,000 miles and am just getting around to posting this question. I've noticed lately that for some reason even though I get heat while mobile down the road, I don't while sitting idle, even after 20 mins. I get airflow, but it's not warm.... that isn't normal for this year on this model is it???

 

No, not normal for any vehicle, temp usually goes UP when stopped because there is less air flow thru the radiator, not noticeably UP, but up none the less.

Could be heater core is getting plugged up, or water pump fins are getting smaller.
i.e. when RPMs are up enough hot coolant is able to travel thru the heater system, when RPMs are low(idle) flow is less so not enough hot coolant can passed thru and heater core cools down.
Try raising RPMs when stopped and see if some heat starts to return.

Google: Back Flush heater core

Vehicle doesn't matter all heater cores work the same, coolant goes in hot and comes out cooler

You need to pop the hood next time engine is heated up and heater is on.
Feel the two heater hoses going into firewall, the hotter one is the IN hose, cooler one the OUT.
Remember which is which or mark them.

To Back Flush you want to send water into the OUT side of the heater core, to flush any debris/plugs out the IN side, to clear it out.

You also could have a Bypass valve in the engine bay, 4 hoses connected, 2 to the heater core/firewall and 2 to the engine.
These can restrict flow as well, it has a vacuum controlled valve, if it isn't opening all the way when Heat is selected, then flow would be reduced.


If heater core has good flow and bypass valve looks good then replace water pump
I would expect engine temp gauge to rise a bit when stopped if it was water pump issue.