This may be the wrong thread, howevr I have a 2006 4500 with an lly and Allison 1000 EVS 5 speed trans, the truck is a Regency upfit with a Link Air ride and all the other bells and whistles that go with the Regency package. Since having the radiator replaced at 3700 miles (fan came apart, destroyed shroud and made a hole in radiator) the trans trouble light has been coming on.
Whenever driving at hi-way speeds and it comes time for the trans to downshift from OD to 4th, the truck bucks, feels like running over a railroad tie!) and the trnas goes into limp mode with the trouble light illuminated. I can cycle the ignition and clear the light and everything runs normal for awhile then it happens again.
I had the truck checked and serviced at an authorized Allison distributor, Stewrt & Stevenson in Albuquerque NM Friday before Labor Day weekend. They found a loose plug and thought it was the problem, afterwords everything seemed fine until yesterday when the problem re-appeared.
Any thoughts or suggestions are greatly appreciated.

You are certainly going to all of the right people.

I would not guess that the upfitter did anything that would cause this problem to be more prominent as a result of the fan explosion.

I'm not a transmission expert, but I have been told that downshifting under load requires higher hydrolic pressures within the transmission than the standard up-shifting.

My guess is that there may be some undiscovered damage to the the transmission cooler (unlikely) or the feeder lines could be dented causing a flow intrusion (more likely). This would cause the transmission, under stress, to starve for enough oil pressure to effect a standard smooth downshift. If otherwise I would say it could be electronic.