to me it seems like that power circuit, but where the difficult part is, is finding the damage. It's easier to damage wiring under the truck than behind the dash, but with these older vehicles anything is possible.
It's even possible the wiring is damaged at the trans or inside the trans.
I believe you can get the cluster out just by removing the bezel around the cluster, In my 2000 there's one screw at the top above the cluster, and two screws at the bottom of the bezel just behind the knee bolster/panel which can be loosened enough to get to the screws, You can completely remove the panel but you'd also need to remove the closeout panel just above the pedals. Maybe an internal cluster issue is throwing the whole circuit out of whack?
You can check the back side of the dash fuse box as well, it's unlikely but maybe someone tapped a wire for an aftermarket accessory. Just grasping at anything I can think of here.
You mentioned hitting a bump may sometimes get the gauges working and makes the trans try to shift. It definitely sounds like a wire shorting out or losing connection somewhere.
You can double check ecu grounds to be sure. there's 3 wires, white with a black stripe, and they ground at engine block on the right side.
But I'm certain a bad ecu ground would cause more ecu related issues.