i just serviced the coolant on my 97 TJ 4.0L and the coolant was full of all sorts of crap, enough debris that i changed it again and had it flushed.
if it hasn't been serviced in a long time i'm sure it's clogged up.
After i serviced the coolant the heater got 50% hotter which is a sign that my heater core was clogged up.
good luck