here is allison 1000/2000 series operator's manual:
http://www.rvtechlibrary.com/transmissions/1000_2000_operators.pdf
on page 14-24, there are clear instructions on how to operate those trannies, including 2500 model. there is nowhere ever mentioning that from D to R must stop at N. The instruction says:
Completely stop the vehicle and let the engine return to idle before shifting from a forward range to R or from R to a forward range.
what is idle? here is the definition https://en.wikipedia.org/wiki/Idle_(engine). notice, idle is not the same as neutral (N):
Idling refers to running a vehicle's engine when the vehicle is not in motion. This commonly occurs when drivers are stopped at a red light, waiting while parked outside a business or residence, or otherwise stationary with the engine running.
so, from d going n before going r will not harm anything, just not required.
