Made a modification to the cart.
My garage floor is uneven, so at any given time there were usually only 3 casters touching the ground. Locking the casters didn't do much to prevent the cart from moving - and I didn't want to chase my table saw around while in use. OSHA would not approve. I was going to buy some floor locks, but they're expensive (about $100 for 4).
I built up some plywood feet and added these stepdown casters. The legs are also braced underneath. Problem solved (for about $40).