Basically, my '97 puts out very little heat. It's warmer than the outside air, but no by much. According to the gauge, it's operating temp is about 155 degrees. I have ensured that there is plenty of coolant, so that's not the issue. Any ideas on what else to check?

Your thermostat might be stuck open. T-stats dont last forever, and they are pretty cheap and easy to replace.

Also check the vents (blend vents for outside air) are working properly.

had this problem with my 2000 back flushed my heater core works great now but thermostat is always the easiest thing to change first

of course this helped mine just something to check out

The 155 degree temp is kind of low and that would indicate the thermostat is sticking open and that's where I'd start particularly if it hasn't been changed in several years. Beyond that, possibly a water valve that is out of "rig".

Update: Replaced the thermostat, engine still shows it's running about 155-160... on my first dive to get it up to temp it did in fat climb up to about 195 or so, but that lasted about 30 seconds and dropped VERY quickly back down to 155. Drove it on errands later, never went higher.

Also, flushed out the heater core. While I did get some debris out of it, it's wan't much. The heater puts out maybe slightly better, but overall still very low.

Probably has an air pocket trapped in the system and a faulty radiator cap so it can't "burp" itself. There is a thread in the Blazer "How-To" section that explains how to burp it, then get a Stant radiator cap. Make sure both hoses at the heater core are hot and approximately the same temperature.