I think I know exactly what your problem is. I ran into this several times when I was an alignment tech. Hyup, it fooled me one-time, but like you, it showed up immediately when backing up and braking.
The new MOOG UCA cambolts do not have enough threading on them. You tighten them up alrighty but there is little or no clamping going on, as the nuts are just bottomed on the threads. It feels tight, but is not.
My solution was to go get some hardened washers about 1/8" thick and slip one on at each end; one under the head of the bolt, and the other under the nut. Never had a problem after that.
