But, that still doesn't explain why my rear heat doesn't work at all
No, I haven't replaced any components in in my cooling system, and yes they both started giving me problems at the same time.
It sounds like the heater core is clogged which will cause restriction on flow to auxiliary heater core. If it was anything else such as blend doors you wouldn't have heat at all so I would still recommend flushing out the heater core which should take care of the problem. You can feel hoses going to heater core, if one is a lot hotter then the other this means it is not getting good flow through it. Let me know if there is anything else I can do to help or if you run into questions, thanks.