On Saturday October 9th, we left home to go to a restaurant, less than two miles away the truck bucked very hard.
It almost stopped, the guy following nearly hit us.
It's done this in the past intermittently, it's been getting worse and more frequent.
I scheduled a service appointment within 20 minutes.
I took the truck to the dealer early on Wednesday Oct 12th. I described the problem in as much detail as possible.
They provided a very nice 2012 Buick Enclave as a loaner. Although it was comfortable, it was underpowered for my taste.

The dealer drove the truck during the day and could not identify the problem.
They drove it again early the next morning and could not identify the problem.
The Service Representative called Thursday and asked if I could arrive early Friday and see if we could create or identify the problem.
In 20 minutes the problem occurred 3 times very lightly, but at least it was visible while the Service Manager was riding in the truck.
The Service Representative called Friday afternoon and stated that they found the problem and ordered a module, it would be here Monday.
He apologized for the inconvenience, and asked to keep the truck over the weekend, I was OK with that.

I picked up the truck today (Monday). It appears to be fixed, we'll know for sure in a few days...

The invoice ($0.00) listed a part and a TAC number PIP4751A.

I used Google to search for the part description.....
.. gmc auxiliary fluid pump control module

After reading through about a dozen items I found this.

http www sandyblogs com / techlink / 2010 / 03 / full-size-truck-two-mode-hybrid-auxiliary-pump-diagnosis.html
http://www.sandyblogs.com/techlink/2010 ... nosis.html

It clearly listed the symptoms I had seen. I cannot express my relief when the description of the problem matched the diagnostic information I had collected.

We'll see how it goes....