One would think that the trans and engine are indeed grounded, but remember there are rubber mounts on the trans and engine that prevent both from contacting the frame, unless of course they are worn or broken. The clamps that secure the tranny lines to the frame would be the only source of a true ground, but then again, if you have an aluminum/plastic radiator, the oil cooler may be fully insulated from the radiator body, not providing a ground, but I see what you're saying.
The radiator has rubber mounts both on top and bottom and the hoses are rubber, so it too is insulated with the exception of the water. But depending on the alkalinity and chemical makeup of the coolant, it doesn't always provide a good path to ground or is a good conductor of electricity.
I think the problem is that with the replacement parts for these trucks being different materials and designs then what was originally designed, some may see some issues with electrolysis that would have not been an issue before [using OEM style parts]. The problem now is that you have plastic, aluminum, iron copper/bronze and steel which all expand and contract at different rates and interact with electricity differently.