Tom, it's a limitation of your (and pretty much every modern) camera.
It all depends on what you're trying to do, and why you 'need' high speed sync really.
If you are trying to balance the sun (by keeping ambient exposure very short) then just use high power from your safari, it should just about be enough, 600ws usually is just for a single person. Bring it in closer and use a modifier that won't eat so much of the light. If you still need more light, use additional lights.
If you also want to match this with a shallow depth of field, then use an ND filter on your lens so you can stop back open
If you're concerned about having a fast enough shutter speed to be freezing motion, then I assume that the thing that's moving is what you'll be flashing anyway, and flash durations should be fast enough on the safari to freeze most things. High speed sync, ironically, would be worse because it pulses the light out over time.
High speed sync is a 'trick' mode that some speedlight systems have, which pulses light out over a longer time to expose the subject during the whole movement of the mechanical shutter, but it doesn't work with any big lights.
Another alternative is to use a medium format camera with a leaf shutter lens, which will allow you to go to a bit higher (can't remember off the top of my head - maybe 1/640th or 1/800th)
I can *just* tease 1/320th out of mine with a cable. With pocketwizards, 1/250th.
A few (and I can only think of the D70 offhand) will sync up to 1/4,000. A Canon G9 (and several other compacts) can also sync at high speeds but you'd really want to use a cable instead of radio.
