chevy silverado remote start

Well, we are on the right track.  It is a Passlock2 error. 

P1631 Theft Deterrent System Password Incorrect 

Does everything else work?  The doors, parking lights, etc.  I would go back to W2W mode and verify the GWR signal is there during a remote start.  ( Maybe even just connect the Brown wire to chassis ground at the same time as I hit the start button.)

Hard to believe you have 2 bad IM01 modules.  What happens when you try to re-program the module?  Does it flash 4 times at Step 2?

When you get to Step 4, did you try holding the key in the Start position for several seconds?  Guess thats why I prefer iDatalink modules. 

Here is another thought.  Have you tried the old "Relays and resistor" method of bypassing the Passlock2? 

Here is a link to the  AudioVox instructions :  http://documents.audiovox.com/700054.pdf   I am sure Omega has it also.  You have the DMM, all you need is a couple of relays and the resistor.  ( The 3rd Ignition is just (-) Status Output.  If you share GWR between the IM01 and the Relays, diode isloate.)