Walter, There are shims available, but if you have driven 50 miles and can observe movement, I am concerned that your axle may be damaged beyond repair. The taper is one thing of concern, but the movement itself can cause a crack in the axle at the inner end of the keyway. If you remove the hub and look closely, a tiny crack now is definitely going to cause you to lose a wheel, which is dangerous as well as costly.
Look at the key itself for a possible cause of the movement also. An incorrect key or incorrect placement of the key will prevent proper contact between taper and hub. The key may also show signs of wear, which would be expected in this case. The key should never take the load, always the fit of the taper itself.
If repairable and with no crack apparent at the keyway, use a flat file to remove taper burrs, and with grinding compound within, mount the hub without the key. The axle nut should be just tight enough to be able to spin the hub by hand, thereby matching hub to taper with the compound. It will not take long to mate the two. Be sure to clean off all the compound, as any left in there can damage your bearings as well as prevent good taper contact.
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