You don't mention 1150 or 1200. Might matter to some of the repliers.
First thing you might try is fussing around with tire inflation up front. Or a different tire entirely.
Next, see if weight distribution of your cargo matters. I might try some start-ups downhill, then uphill, to see if the shake is better in one direction than the other. If better downhill, then I'd shift some of my ballast forward.