After 25,000 miles of driving and a lot of towing I am very impressed with the Allison. The only unusual shift I have felt is under WOT from 4th to 5th. It is consistently a hard shift versus all other times. It feels like power drops off as well. Is this normal?

At WOT in 4th the converter clutch is on and stays on during the shift. That will make it more pronounced. Try reseting the 4th to 5th TAP cell. Only a Tech 2 can set them individually. If your other shifts are acceptable, leave those cells alone.

mike