For a DD Rubicon express superflex or Currie Johnney Joints are your best bet.
RE Superflex Joint
Currie Johnney Joint
They are the ones that I refer to as "Flex Joints" in my first post. As you can tell from the pictures the designs are virtually the same. I can't tell you if either is better but I haven't heard anything negative about either.
My best guess as to what you are refering to as ball joints it what nTh degree uses on their long arm. They call it the gyrojoint.
Its essentially a threaded ball similar to a hitch sandwiched between two bushings. Provides isolation from the road and has a huge range of motion. Difference is these are used on the frame end unlike most of the other products. The Nth kit has a crazy amount of flex (just read the linked article) but apparently has a better ride than stock. Guess it helps when the designer worked for DC when they were working on the TJ.