On 2004 Trailblazer, when air conditioning is turned on car does not idle right almost cuts off. no problem at speed, only idle & slowing down.

normal idle speed too low ( should be around 800rpms minimum)
idle compensate circuit is not operational to speed the idle when the ac cuts in--run the fault codes

have your IAC and mass air flow sensor checked surging can be a problem in either one IAC( intake air control) or idle air control.
does the car have any obd2 codes? have it scanned for codes

THIS IS NOT A PROBLEM.

its not being told to speed the idle up under load ,now on the back of the steering rack just behind where the steering goes into it is a sensor that increases the RPM on idle when steering is turned but i have found that if this sensor isnt working not only does steering slow engine down but also AC ?? why when they are not related i dont know i just know it does ??

The first thing to do is the throttle body needs to be cleaned, grime in this area prevents the throttle plate from closing completely and allowing air to enter, making the air/fuel mixture lean. The highway problem be cleared also.

If the air conditioning is on, then the engines idle will increase when the air conditioning compressor cuts in, and slow down again when the a/c compressor cuts out again.
If your model has a heat switch, if this is on, then the engine will idle faster, until the engine temperature reaches about 50 deg C, then as the glow rail switches off, the idle also drops back to normal.

Needs to have the throttle body and throttle plate cleaned. While cleaning is being done, disconnect battery for about 30 minutes, this will cause computer to relearn to the freshly cleaned throttle body.

Here is the common cause of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake

clean air filter and adjust idle speed, (increase)
rev it high then instantly release it, if it stops, increase more the idle speed,

Here are some of the causes of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.