Update: Took the truck into the transmission shop Indy recommended. They serviced it (without flushing) and refilled with +4 fluid. They also added Lubegard to remove the glaze buildup on the clutch pads (???). They told me it was the cause of the grind going into overdrive.
After driving it for a week, the grinding is gone and it shifts smoother.
I've read a lot of "anti-additive" rhetoric, should I be concerned about the Lubegard?

 

Well,

If it's helping then great.

 

If Indy recommended the shop, I'm pretty sure they know what they're doing. Like hydra said, if it helps, use it.

 

Exactly what I was thinking. I'm happy with it.

 

Bring it to a good tranny shop!!!!!!!!!!