When I was at the dunes my 300EX handle bars would shake real bad about mid throttle in 1st and 2nd gear. At low throttle and other gears it would not shake. I recently put tire sealer in my back tires (sand skate II) because they were leaking. Do you think this could be the problem or any suggestions?
Thanks

If it was due to the sealant in your tires, it would do it regardless of what your gear your in.......only the speed that your riding.

Look for loose or broken engine mount bolts, loose steering stem clamp, worn tierod ends, bent axle and front wheel bearings for starters. Then move onto other parts of the chassis for loose bolts or worn parts.

Did you notice this right after you unloaded it, or after you had been riding a while?

It started after we unloaded it. I put the dirt tires back on and road it around my house and it does not shake. At the dunes with sand tires the handle bars shake really bad.???