If you have the screw/compression casters and your wheels are wobbly on your M55/M48 (it may work on others), it may be that the rubber inside that is worn out or soft. I got replacement 7/8" compression rubber and the spacers on Ebay and it made a big difference. Just be careful not to overtighten or you can crack the legs. Also, when you take out the old ones make sure you only loosen them enough to pull them out; don't screw them out all the way or the rubber and nut will be stuck up in the leg. Check out a picture of an "expandable stem caster", it will make more sense what I'm talking about.
Expandable Rubber Stem Applicator. Size B for 7/8" ID Tubing
Mine came with nuts that didn't fit into the legs so I reused the knurled nuts on the original casters.
Your mileage may vary, but for about $22 with shipping it may be a cheap and quick fix if your casters are otherwise in good shape.