I have a 94 SSE that doesn't idle smoothly and runs rough in the 1500-2000 RPM range,it runs fine when accelerating. I've replaced the plugs, wires, fuel filter, air filter, PVC valve, and O2 sensor no improvement.

fuel injectors? ICM?

check the ohm on the coils...get a ohm tester take each coil off...and put the ends in the bottom...i dont know the exact numbers its in tech info but if one is really far off from the other than you might have a bad coil!!! if i were you i would start with that!

Ignition wouldn't be the problem in that RPM range (assuming it'* ok below and especially above that range).

I strongly suggest at this point (having done a full tune-up) that you get a diagnostic tool on that car and see what an experienced person can come up with.

If you add your location in your profile, we can find out if there'* a member near you with a scantool that can help.

Could it be something as small as a bad tank of gas.

Here'* what GM says to do:

Rough, Unstable, or Incorrect Idle and Stalling
DEFINITION: Engine runs unevenly at idle. If severe, the engine or vehicle may shake. Engine idle speed may vary in rpm. Either condition may be severe enough to stall the engine.
STEP 1: Was the Powertrain On-Board Diagnostic (OBD) System Check performed?
If yes go to Step 2
STEP 2: Perform a bulletin search.
If a bulletin that addresses the symptom is found, correct the condition as instructed in the bulletin.
Was a bulletin found that addresses the symptom? If no go to Step 3
STEP 3: Was visual/physical check performed?
If yes goto Step 4
STEP 4: Check the PCM grounds for being clean, tight and proper routing. Refer to Ground Distribution in Electrical Diagnosis.
If a problem is found, repair as necessary.
Was a problem found? If no goto Step 5
STEP 5: Observe Long Term Fuel Trim on the scan tool.

Is Long Term Fuel Trim significantly in the negative range (Rich condition)?
If yes goto Step 6. If no goto Step 7.

STEP 6: Check items that can cause the engine to run rich. Refer to Diagnostic Aids in DTC P0172 Fuel Trim System Rich .
If a problem is found, repair as necessary.
Was a problem found? .........

It goes on & on......BUT FIRST YOU NEED TO GET THIS VEHICLE SCANNED!

check coils....lol...my car ran like butt when i had a coil starting to go...

That CANNOT be ignition related, if the problem is as he states. A bad coil will be bad at idle, midrange, or high rpms. It may be more noticeable at high, but nevetheless, ignition components are BLIND to engine rpm.

Considering the midrange rpm problem, with no apparent high end problem, I would rule out ignition as stated before.

Get the car scanned, and in the meantime, do as Jeffrey suggested, and look for a broken/worn vacuum line.