If memory serves me correctly I had a similar truck with a similar concern and it did end up being the instrument cluster. But it was a while back so I am not 100 percent sure.
I buy it by the gallon. I saved a whole bunch of the single shot containers the advisor used to sell with injector installs. Fill a bunch of and put them in the box.
2 "shots" per tank.
I twisted up a batch of "apple pie" over here a couple months back.
It's dangerous stuff. Can't even taste the booze in it.
And then you try and stand up after 3 shots. And promptly sit right back down.
Try transmission shops, they have some pretty heavy duty wash tanks.
I've had them dipped before and they come out nice but not as nice as that one up top there.
When are tours available of the glass case? Suit and tie required, naturally?