They are "horizontal axle dampners". "Quad shock" is common slang for them.
The "Quadra Shock" rear suspension, introduced to eight-cylinder and turbocharged four-cylinder Mustangs and Capris in the middle of the '84 model year, included those axle dampners.
They can be purchased at any auto parts store...
Gabriel part # 14039 (called a "steering" dampner)
OR Koni, KYB, or Monroe...
http://www.allfordmustangs.com/forums/461800-post2.html
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'83 GT, '84 LTD LX, '85 LTD LX (parts car), '96 B2300 SE, '99 CVPI