""I have tuned the carb while in park, but I have not done so while in drive.""

Since you have an auto, you need to complete this first before swapping parts. Drive the car and get it up to running temp, then either apply the parking brake, or ask a partner to put it in D, then start with one of the metering screws. Turn one of the MS's inward about 1/8 of a turn and wait about 30 seconds or so. Then reset your idle speed if needed and check your vacuum reading. If you gained in vacuum, then make the same adjustment to the next MS. If the vacuum reading got lower, then turn the MS back to where you started and then 1/8 of a turn outward and wait another 30 seconds or so. Once you go back and fourth between the MS's 2-3 times, you may need to reset your idle rpms again. But you should start to see a difference in your vacuum numbers (after turning the MS's in or out) unless it was tuned correctly to begin with.

Note: Make sure that your MS's are set the same distance out before your start. Try to keep them the same turns outward. I like to take a permanent marker and I make a mark at 12:00 on each MS after I lightly snug them inward. This way I have a "visual" of how far out each MS is while I am tuning.