THe shifting of the 1000 series Allison in T/H is speed dependant. It will not shift into 5th in T/H unless speed is over 60 mph. I know what You are saying about the upshifting to the highest gear. I have the same issue with the 6 speed Allison (I think it's a HD 4060P) it shifts to 6th when throttle is released, as soon as the Jake comes on it immediately downshifts to 4th. This truck does have a 505 Cat pulling it along
. The smaller trans does not do this. It will hold the gear that fits the speen, when the brakes are applied it will downshift and grade brake to slow or maintain speed.
John