Cartridge replacement fixed it!
Thanks for all the advice! I got the Chateau Posi-temp Tub/Shower Replacement Cartridge (Moen part#1222) at Home Depot. The Home Depot guy said I didn't need the cartridge puller tool because a twisting tool (white piece) that already came inside with the cartridge would do the trick.
However, I read on many sites and on this forum that sometimes that thing doesn't work, so I bought the pulling tool anyway...just in case.
Got home and followed the instructions provided with the cartridge. So far, so good, until we got to actually pulling the freak'n thing out with the little white twisting tool they supplied. I'm not what you call a "petite man"...so I kept pulling on that sucker and it wouldn't budge. The wife thought I was going to yank the pipes out of the wall, so I went to the shiny pulling tool I bought and a few minutes later, out slides the cartridge.
Put everything back together, turn the water back on, crossed my pliers, feet, whatever...and asked the wife to turn the shower on/off for testing. To my relief, NO MORE LEAK!!! Many thanks for all the advice!