Heating issues??

I have a 2001 Blazer and I am having a problem with intermitten heat. I can turn the blower on 1 or 2 and have heat if it's above freezing outside. I have no heat on 3 or 4 and if it's below freezing outside. Has anyone had this problem? Could anyone give me some ideas to try to solve this myself. I am low on funds for repairs and would like to resolve this myself if I can. Thanks for any suggestions that anyone has.

It could be that its so cold coming into the heater that the air doesn't have time to absorb all the heat at higher fan speeds.  In other words, the slower the fan, the hotter the air; the faster the fan, the cooler the air. 
You may need to do a flush of the heater core and see if that helps any.

Check your heater hoses and see if they are both hot. If one is hot and the other si cool or just warm, then you have a partial blockage in the heater core and it should be flushed out.
Make sure that your coolant is not low in the radiator.......check it with the engine cold.

I agree, sounds like a partially clogged heater core.  Could also be a low coolant level, but this is typically combined with a rushing water sound under acceleration.

I looked under the hood and to me it looks like the hoses go inot this cylinder thing and then into the heater core. So how would I flush the heater core?

just move south lol

Disconnect the two hoses that go into the heater core from the block.  Hook a garden hose up to one hose and put the other in a bucket.  Turn the garden hose on and let the water flow through the heater core, switch the garden hose to the other side and do the same.  Do this until the water is clear and.  Hook the hoses back up normally and check your heat.
 
Should take about 10 minutes
 
Brian