electrical gremlins drive me crazy; sure seems like a ground issue.
At this point, I'd either be adding pigtail ground wires to the taillight plugs to see if it helps or doing the Big3 wiring to see if it helps.
I don't recall, did you already replace the IBS sensor on the ground? isn't that little control freak on the primary ground wire?
Also, for lack of ideas, have you tried disconnecting all the accessories from the ground?
Hmmm... Ripley is pretty smart, if she can find the monster, set her loose on the truck! Ripley, get the electrical gremlin!