This thread, while about a different subject, will give you some good advice about jacking and supporting your trailer to work underneath it.
When I did mine, I did one side at a time but LAdams did both sides at the same time. You will need some jack stands for support after getting the trailer up off the ground and leave the trailer hooked up to the truck for additional stability but do not hook up the WD bars.
I used both a floor jack and a 12ton bottle jack to lift the trailer. The bottle jack works best to get the thing up enough to put the stands under the frame.
Good luck on your project.
Barney
