YOu can retain your existing boomerang and simply replace the bushings (boomerang to body ones), and the Diff balljoint,; or buy the compelete Boomerang assembly from a place like IDParts:
https://www.idparts.com/rear-upper-c...76-p-6147.html
or others online.
The times I had to change my Upper Ball joint on the Boomerang, the first two times I just changed the balljoint and it is a bitch of a job to get out and press the new one in. The last time I simply bought the whole boomerang from IDparts, and you get the body bushings included.
I see IDparts wants $129 USD, and I don't thin the shipping was too bad, bu the exchange rate is pretty bad now. Still it saves alot of time, and certainly saved me money.
You do need to be very careful when changing that rear boomerang, as the bolts that secure it to the body screw into captive nut fittings which are spot welded into pockets in the body (and you cannot get a tool onto those nuts). I recall a certain KJ owner in WA who had to cut out the rear floor area to access a nut in the captive pocket which he had busted loose with a ratchet gun, so he could reweld it in the pocket.
Do your research before you try it, and only use hand tools.