Excellent pictures BUT they scare me because the springs have already dropped a bit. As Nagrompj said DON'T get under the trailer right now. If those nuts on the U bolts come loose the frame could drop suddenly.
Nagrompj had excellent advice in jacking the frame and using jack stands so the frame will not drop on you.
You only need to support the side you are working on. If you want to do both sides at the same time you will need 4 jacks and 4 jack stands. Make sure the jacks and stands are rated for at least 1/2 the total weight of the trailer. The jacks and stands need to be as close to the ends of the springs as possible so the frame rails don't flex. Trailer frame rails are no where near as strong as the rails on a car.
Be safe and let us know how you do.
