Idle rough when brake applied in gear.

Were you able to find the cause?

I should add, it only does this while the (automatic) transmission is drive, if I put the car into neutral or park while the brake is pressed, the idle goes to normal and the vibrations go away.

Upon further testing I was able to cause this low idle condition without pressing the brake pedal. I found that if I use the emergency brake to slow down to a stop the engine will experience this low idle situation when the car gets below a certain speed (somewhere under 10MPH - I didn't note exactly) but is not yet stopped. So this seems to be happening in two instances so far; 1) stopped while in gear with brake pedal pressed, 2) very low speed but not yet stopped with either brake pedal pressed -OR- the e-brake being used. I think this would eliminate a brake booster vacuum leak (and I tested around the vacuum hose but couldn't find or hear a leak anyhow) since I can replicate it without using the power brakes.