First, let me say that the people on this forum are incredibly knowledgeable and helpful - awesome!
I tried some of the suggestions that have been posted. Here's what I've found:
First, when I tipped the machine forward, the wheels were able to turn freely when in neutral. When I then put the wheels back on the ground, I got the same result. Turned about a 1/4 revolution, then stopped. It just doesn't have the power to turn the drive wheels.
After taking off the belt cover, I noticed that when I engaged the drive clutch the Attachment Drive Idler Pulley wasn't engaging (see pic below - the pulley on the bottom-most left). I believe that pulled is supped to press on the belt to create tension and turn the plate. It seems that some sort of linkage to this pulley is either broken or stuck or something.