This post is the first I've heard of the issue , but the grounding of the differential makes sense; because all the springs , bars , etc. in the suspension are bushing mounted, so no existing ground path from the housing to the frame . Although I'd think the sensor wiring would have it's own ground path.
The grounding kit may have been little more than a length of wire bolted to the differential and with enough length to allow for full suspension travel , then properly grounded to the frame.
Might be worth the time to hook a set of jumper cables in place ; from the diff to the frame ; (check for zero ohms resistance) , then a quick road test.