Jacking up a trailer isn't going to hurt it. If you had a flat while on the road you'd have to jack it up to replace the flat. Place the jack under the axle where the spring hanger is located. If you put the jack on the frame you'll have to extend your jack more because as you jack up the frame the axles will drop down until they limit their bottom movement. By placing the jack on the spring perch the axle will rise as you jack and you won't have to jack as much. To get both axles up jack up one, block it up, then jack up the other. Place your board under both tires and then repeat the process to lower.
