Are the shackles moving when it flexes. They may be too tight. Use nylon lock nuts and cut the torque to 45 ft/ lbs. see what that does and yes when it is on the ground not in the air.
89YJ 2.5L TBI, AX-5, NP231 SYE, Ford 8.8, Yukon Grizzly, Tom Woods Shaft, 33" BFG T/As, 4" lift, Warn VR10000s.
AF Jeep Club Tail #184