I think it is called a pin spanner. The link below has a photo of one. Remember, Snap-On tools are at the top end of tool pricing. Other companies should make such a tool, maybe even Sears. Make sure you get the right pin diameter for the holes in the nut, this can vary. The wrenches are also used on boats fuel and water caps, machine flanges, etc.
You might check with a shock absorber dealer, auto parts house, tool house, on line shock and auto parts websites. You have a replaceable strut. Meaning you need only replace the strut and not the complete lower end of the assembly. Individuals such as TN members buy these types of struts and replace them themselves all the time. They usually upgrade to some kind of performance type strut. As such they would need a tool to do the job and the tool should be very common.
Don’t let the nut completly come out or the strut assembly could come apart. If all else fails use a small diameter punch and a good size hammer.
The link is from the on-line Snap-On tool catalog.
http://buy1.snapon.com/catalog/item....re&dir=catalog