Sounds to me like your rear brake shoes are worn...they'll often GRAB when you hit the pedal at speed if they are worn.
Easiest thing to do is take off a rear wheel and have a look at the shoes. Best way to get the drum off is to get 2 of the correct size bolt (sorry, can't remember exact specs) and wind the drum off.
If it's not your brakes, then look at your suspension...check both ends of each spring to make sure they're attched where they should be, and that the U-bolts where they attach to the axle are tight. Also check if a shock hasn't come loose.
Hope ya find the prob soon.
P.S. If ya "Hit the Pro's", be careful, the ones in Darlinghurst here in Sydney would prolly beat ya to a pulp.
