I had this happen to me on my '97 supercab 4.6....definitely the heater core.
The dealer wanted around $1100.00 to replace the $37.00 core! 99% labor for this repair. I decided to tackle it myself with a Haynes manual at my side. Took 2 full days - you have to remove the entire dash to get at the air box that houses the heater core. A huge PIA......I paid the grand to have it done on my wifes '98 Expy when the hot/cold mix gate failed iin the airbox. I didn't need the back pain again.. I'm 6'3". I hope there are some other threads on this site, because it is a big job to take on. Good luck!
This forum is owned and operated by Internet Brands, Inc., a Delaware corporation. It is not authorized or endorsed by the Ford Motor Company and is not affiliated with the Ford Motor Company or its related companies in any way. FordŽ is a registered trademark of the Ford Motor Company