97 TJ Sahara. Here are the problems:

1. I've got coolant leaking from the floor vents on both sides. Replaced the heater core 2 years ago, so it better not be leaking again. Any ideas?

2. Defrost doesn't work. Not sure how the heater operates, but when I turn it to defrost, I hear a bang, like a flap has closed and I hear the pressure of the air trying to flow, but it can't get out. Hard to explain. All other settings (blow straight out, and floor vents) seem to work fine.

Sounds like you have a vacuum leak. Check your vacuum hoses around the firewall/heater area.

1) make sure that the heater hoses are not leaking at the heater core, otherwise, the core is bad.

2) There's a plastic lever for the air door on the left side of the HVAC housing that breaks. You used to be able to buy just the lever, but I think it superceded to a whole door/lever assembly. If you find it broken, the part that broke off will be inside the door in the housing. A long drywall screw will thread inside the broken part so you can extract the broken piece. If the new part is only available as a complete door, the housing will have to be removed and the case halfs split.

Hope this helps!

P.S. If all you lost was vacuum, but the housing was still operating properly, the system would default to the full defrost position, full heat as a safety measure.

ok, I checked the two hoses that go into the heater core at the firewall, they are fine. I noticed that about a foot below those two hoses, on the firewall, there is a connection for a hose. Water/coolant is leaking out of that. Should there be a hose on there? Where would it go? I don't see anything disconnected.

Sounds like that is a condensation drain tube and should be fine as is.

There should be a vacuum line running into the firewall. If that is in place and no leaks, follow Marty's steps about internal to the heater core.

Not seeing the vacuum line. How big is it and where should it be?

I havn't looked at my late model Jeep but all before have had the vac line from the back of the intake manifold almost directly straight into the firewall.. So my point is that line isn't near the heater hoses. It is pretty easy to see if you look at the entire firewall.
If you don't see the line then there should AT LEAST be a nipple on the firewall where the line has fell off..
Big JIm <img src="/forums/images/graemlins/pfft.gif" alt="" />

If coolant is leaking out of the condensation drain tube, and the heater hose connections are tight, the core is bad.