abs light flashes and speedo drops

[POWER-STROKER250]

quick question i have a 2000 psd 6 speed 2wd with rear abs only and my question is, when i'm going down the road and i hit bumps my abs light will come on for like a second and my speedo will drop towards zero mph and than go back up to the speed i am currently driving at. i have already manually tested the wiring all the way up to my rear speed sensor by shaking it and having someone watch if there is any fluctuation in the vss signal but there's not, and i have also replaced my vss sensor with a borg warner speed sensor. i have heard that its best to use motorcraft only sensors but i'm not for sure on that. thanks for the help any help would be great