The gauges don't receive their ground from the dash. They receive their ground from the senders.
I would say that it is just a coincidence that its a bad gauge. Or at the least a bad connection on the back of the gauge where the circuit board is. Besides the YJ gauges are mediocre at their best. The problem prone circuit board is the common area to start with when having gauge problems.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...