What kind of dually did you get?
I learnt to whip, or as well as I can whip, by doing a stoppie, then twisting my feet, pulling the bike that direction. I can pull a bit over 90 degrees now, which isn't too much but plenty for a whip. Eventually move up onto smaller jumps you can try this on, small enough that landing with the rear sideways really wont matter, but big enough that you'll have time in the air to pull the whip off. Just go from there, thats how I learnt, but I know there are specific threads on how to whip somewhere on here. Practise twisting your feet both ways, I can only whip to my right, can't do it to my left at all.
Jumping higher I find just preload heaps into the jump and let the suspension boost you up off the lip, but also keep your weight a bit rearwards as to launch the front wheel up, but make sure you correct the weight placement in the air so you land both wheels simultaneously, or close to it.
Rock gardens are simple enough once you've got the hang of them, I like to keep a fair bit of my weight towards teh rear to help the front wheel make it's way over the rocks, and make sure you have plenty of control over your rear brake. Probably not advised but I like to lock up my rear wheel over some sections as to make it slide when I need to change the direction of the wheel. I ride an Ironhorse Sunday, and the low BB can make rock gardens a bit of a bitch at times if you fuck it up, so make sure you've checked it out first, when you're off the bike, and find a line that looks rideable. Sometimes you'll find going faster can help to, helps the wheels no get snagged inbetween rocks, which can have interesting results.