Making a diversion back to the O.P. ...

Riding out of the saddle is still possible, but if you're riding with a load you don't want to throw the bike from side to side as much as you would without the load. After all, your 18 pound road bike has been replaced with perhaps 60 pounds of bike + load. At the easy end of the change spectrum, try to keep the bike a bit closer to vertical. That may require a bit more upper body strength, or maybe it's just a question of how far you lean the bike before you start limiting the lean. At the extreme end, pull the loaded bike slightly to the opposite side, so your weight on the pedal is closer to centered over the tires. With a bit of practice, this new skill will find you ready to start weighting the opposite pedal when the bike is leaning toward the pedal that's now at the bottom of the stroke.