Is the "not blowing strong on heat and is strong on cool" happening regardless of setting it to defrost, vent or floor?
If so, it does sound like something is blocking the airflow through the heater core. Hopefully someone will give you a better way to fix that than to set it to vent and heat and use a leaf blower to blow into those vents and hope for the best. Come to think of it, if your TJ never had AC from the factory, you may be able to remove the blower fan and use a mirror to see what's inside the heater box that is blocking the air flow and get rid of whatever is in there.
That gage is reading low. That is unrelated to your fan strength issue. It could be caused by a few things, the gage itself, the sending unit or the thermostat. I think the thermostat is most likely the problem.
That is all for now. 