So, another test run and I isolated the problem:
Not enough traction on the drive wheels for the muddy conditions. I have several options. Wait for conditions to improve (either freeze over again or wait till spring/not so wet), put down some scrap wood to drive on (found some 4x8ft on craigslist for $10 each), bring Marduk to a parking lot, cut up the rear motor mount so the mech doesn't tilt forward so much, or add a weight rack to the rear. I have a few tractor tire weights and misc cinder blocks so I'll be trying the last one first.