Instead of mounting to the frame with p-clamps,use 3/16-1/4 U-bolts to capture the tube from behind.Use the strenght of your tubing to your advantage,don't isolate it from you bicycle frame with little tin clamps.You can put rubber or whatever between.You'll have enough clamping force to crush the frame if you want.You'll have to tear the rear triangle off of the bike for it to fail.Simple,solid.

With a rod end(heim joint) you'll get roughly 30* movement total,depending on the bolt head,washers,ect.Should be fine for a trailer but they are not really designed for that type of movement.

http://www.lovejoy-inc.com/,you'll find what you need.You can find their stuff all over the net,been around for 100 years.You can get a small u joint or cv joint.They're the same price as a good rod end.For the big bucks,you can have stainless and needle bearings....Yeah!