I know you said you have a good ground on the regulator but double check with an ohm/volt meter and make sure.

Next, I would take a 10ga or higher jumper wire and go from the alternator CASE to a known good ground (or better the negative battery terminal). When you connect this wire, see if your charging voltage doesn't go down a bit. Ideally you want a charging voltage of ~ 14.5V or so. You aren't too far off but 15V is right on the edge where you may be blowing bulbs prematurely and hurting the gauges over the long term.

If the charging voltage does drop when the jumper is connected, your alternator case itself has a poor ground. To test further, take a second jumper wire and connect the VR body to the alternator case with these 10ga jumpers. This will ensure you have a good ground and rule out grounding issues.

Let us know how it goes and good luck!