Does this happen on every downhill, or only on downhills after you've been riding awhile (especially after long climbs or on super long downhills)?
I had those cramping issues in the past, also. Strength training was not my issue. If anything, my overall leg strength is LESS now than it was when I had those cramping issues. For me, it happened after riding for awhile, so I was tired when it happened. Strength wasn't the problem. Stamina was. The only answer was to just ride more. Take a break when I started having problems (electrolyte drinks can help if part of the problem is because you've sweated away your salts), and just keeping it up.
If it happens early on, strength work would also be a good idea, but if this sort of thing only crops up after you've been riding awhile, I don't think strength work will help you.
Make sure you're replenishing electrolytes on long rides. If you start cramping, make sure you rest and stretch, and keep riding.