The shelves that were under my bench were almost impossible to get to, I pretty well had to get on my knees to reach anything so the logical solution is to have the shelves come to me.
Step 1: I Used
I used about 10' of 2x2, 10' of 1x2 , 4- 14"x 22" of 1/2" plywood, 4 fixed casters, drill, circular saw, speed squares and screws
Step 2: The Shelves
I screwed the 1x2 under the ends of the shelves and cut half-lap notches with my circular saw. Just cut on the lines, then as many cuts as you can fit between them. Then use a chisel or screwdriver to break out the pieces and run the saw back and forth sideways in the cuts to clean them up. I clamped the shelves together so I could cut them all at the same time
Step 3: Casters
The smallest casters I found are for appliances so they have bolts sticking out of the top instead of a plate so I had to set them just inside the 1x2's on the bottom shelves. I centered the holes as close to the 1x2's as I could, made sure the casters were square so the shelves run straight and bolted them in.
Step 4: Putting Them Together
After I cut the half-lap joints in the 2x2's the same way as the shelves, I squared and clamped the shelves to the 2x2 legs, predrilled and screwed them together. For the french cleat sliders I screwed a straight piece of plywood to a scrap 2x4 and cut a 45 about 1 1/2" from the edge then flipped the 2x4, screwed the plywood to it again and cut off the rest of the cleat. I screwed one half to the tops of the 2x2's. When I cut the cleats, I made them about 6" too long and cut off the extra to use on the hangers. The cleats don't actually carry any weight, they are just there as guides so the shelves run straight and balance the shelves so I only needed 2 casters under each.
Step 5: The Hangers
I rolled the shelf under my bench so I could see how big to make the hangers. I cut 2 pieces of 2x4 (1 for each shelf) and notched it out big enough to hold the french cleat. This way the cleat can't come apart. Screw the 6" cleat to the hanger, slide it onto the shelf cleat, and screw the hanger to the bottom of the bench. Slide the shelf and pat yourself on the back