Just dropping a line to thank all on this thread
I was having the same low temp/blowing cold air problem.
Changed thermostat: No change (see note below tips to make changing t-stat easier)
Decieded to change heater core: Problem solved
I couldn't believe how easy it was. See post in following thread for really easy directions;
http://www.explorerforum.com/forums/showthread.php?t=184090
$22 Autozone heater core and it started started blowing smoking hot air, temp guage is steady at 1/4 into normal zone.
In addition, the hoses at the old core were so stuck-on(probablly been on since new) I had to cut them to get them off. Luckly there was enough slack hose to reach the new core. If you have to cut your hoses to get them off, check to see if there is enough hose to reach new core. If there isn't enough hose, get about 1 foot of extra hose and some splice fittings to reach the new core. The locaton of the other ends of those hoses is very, very tight and getting them off would be a b***ch. Splicing-on will be way easier!
Note on changing t-stat: As discussed elsewhere there are a couple(1 in particular) bolts on the housing that are very hard to get at.
After getting the belt out of the way, the tensioner moves a little out of the way. Next, remove the top bolt that holds the tensioner assembly in place; take a hammer and punch/large screwdrive and gentlly tap the assemblly up and a little more out of the way, it will rotate at the other bolt that's behind the tensioner pully. This will give about 3/8" that will make all the difference in being able to get at that bolt(you'll know the one). Now, with a screwdriver and using head of the bolt to lever on, gentlly press the small wiring harness against the t-stat housing, This will get the harness out of the just enough so it will not be in the way. With this, the bolt came out pretty easy
When putting the bolts back in, I used a small bit of duct tape to hole the bolt in the socket; most of the tape was on the socket so once the bolt was started back in, the bolt would disengage from the socket.
A few other good tips I got from the forum article:
-Tape the sockets, extensions, wobble drives on so the don't fall off.
-Have a magnetic retriver handy when you extract the bolts
For complete description see:
http://www.explorerforum.com/Singleton/web/pages/thermostat.html
Thanks again for all who helped