I haven't tried that specifically, but I know what you mean.
I did measure resistance, though. The grounds of each RCA jack measured .5 ohms or less to the cage of the head unit.
On
Crutchfield's advice, I also trying grounding the cage separately to the dash with no luck.
The only thing that's worked so far is running accessory power directly to the battery. I hope to install noise filters on both 12v inputs later today. If that works, I probably won't use it as a long term solution. Instead, I'll probably install another fuse panel and run power directly to the head unit for constant power and via a relay for switched power. At that point, the only connections to the factory harness would be the antenna, back up camera, the reverse signal for said back up camera and CANBUS signals used by the PAC RadioPro (included with dash kit).
At that point I'd say that my problem is under the hood. Since my truck is still under warranty, I've already spoken with the local dealer about investigating "dirty" power. Predictably they said they couldn't help since I'd installed the stereo and therefore might have caused the problem. I'd need a very convincing case to demonstrate otherwise.