We all know it sucks when you just don't have enough room in your shop.   Being able to quickly move around your machines, tables and shelves in your shop can make it easier to get room to work on your projects.  When you are done with your project you can move everything to make room for your car if your shop is in your garage.  I had the same space problem, so I began adding wheels (casters) to most of my shelves.  I have a lot of shelving in my shop that I use as drying racks for the wood slices I cut. 
Here is an easy way to add wheels to those cheap plastic shelving units.
Turn one shelf upside down and cut 4 pieces of wood that fits tightly between the pole holes and clamp in place.  Then cut four 45-degree corners, and glue and screw them to the other pieces of wood as shown.

I did not fasten the wood base to the bottom plastic shelf.  I just made sure that the wood parts fit tightly, and then added the 45-degree corners to hold those together.  The weight of the shelves and whatever is on the shelves should be enough to hold the wood base in place.  There are many different types of plastic shelves, so you will have to make it work with whatever brand you purchased.
Also, be sure to get large enough casters to hold whatever weight you expect to add to the shelves.  The larger the casters the easier it will be to roll the shelves around, especially if you have wood chips or sawdust on the floor.
​Be sure to pound in the poles straight and tight.  Use a piece of wood so that you don't wreck the plastic.