Do you have an internal frame? Or have you just glued and screwed the edges of mdf panels?
You should be concernced about the cab if it racking instead of rocking. If it's racking, it's likely to come apart at the joints one time. It's only going to do it for so long before it fails structurally. Racking would be like if you took a square, pushed sideways on the top, the bottom stays in palce, and the top slides over until you have a parallelogram.
If it's tipping on the castors, you should probably be worried about it falling over.
bracing the cab might be as simple as screwing a couple of metal straps inside diagonally. I've used these metal straps or cables to brace docks as such things before. It's quick and easy even if it's not the best solution.