If you haven't already, read this.
If you have some time on your hands all of it is good info to have tucked away just in case but section 2 (blend door), specifically paragraph "b" seems like it may apply to you.
Failure of these HVAC doors seems fairly common on the Ram trucks. I didn't think about this before because even though I have read through that entire site before, my problem is the recirc door. I haven't bought one of these kits yet but I may eventually.
I think your problem is either the blend door or a clogged heater core and I would lean toward the blend door. Maybe trying the heater core and hose test as described on the website will help you determine. Good luck!