I've actually thought about this some. This might be what is called a "ground loop." Some ground systems are fussy. You cannot go wrong with what is known as "single point grounding." This basically is just what it says...........everything is grounded to one central point.
This means if the battery is in the trunk, you need a separate ground all the way to the engine block, say, the rear of the head. Take off from a bolt nearby and jumper from there to the body.
If the battery is up front, ground to block as per usual, and take off a bolt nearby and jumper to the body, say, with a no4 starter (ring to ring) wire.
Run an ISOLATED ground from that very same single point on the block to the ECU.
The only other thing I can offer is to dig up an original OEM manual, and review how EXACTLY this was grounded originally. In other words trace down just where the ground went originally.
"Ground loops" can be VERY troublesome, in just this sort of situation........where you have unknowns. In an OEM designed and tested system, this has (most of the time) been checked, checked on, designed out. Audio systems and instrumentation are another example. Anything that uses fussy, delicate, low level signals and sensors is prone to this kind of thing.
I don't have acess to these late OEM manuals. If anyone knows (maybe one of the truck forums) where you can download one, that would be great.