There's a couple of different ways to make them but for your basic arm, you'll need some tube (2" x 1/4" DOM preferred), a bushing, a flex joint with a threaded shank and a tube adapter.
You can order the parts through almost any 4x4 fab website. Ex. Ruffstuff specialties, Ballistic Fab, Barnes4wd, Rubicon Express, etc.
The costs for the joints and stuff adds up quick...you may be pretty close to the price of some used arms from another forum member thats going to LA's.
2000 WJ 4.5L - Stroked, Locked, & Long-Armed
Looking for 6" WJ springs!
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