This is my landlord's mower, not mine.
Anyway, I gave it a full once-over and did the following maintenance for him:
Sharpened and balanced blades
Cleaned and oiled deck
Replaced drive belt (had to remove deck, hydrostatic clutch, trans fan, pulley, and bend the engine pulley keeper tabs back just to get this belt off.)
Replaced mower belt
It's due for new blades, but I think we can get another year or two out of these after sharpening. One of the spindles' hubs are almost rounded off, and the hydrostatic trans is leaking fluid. I checked but didn't see anywhere to add (I guess it's a sealed unit. FAIL.)
This thing runs like a new machine now. The deck vibrations are gone as well.
Hopefully we can at least get through this year without any more repairs.