Most likely burnt clutches, Not a expert on Allisons, But worked on a few...That code is not good!
Info on the code.....
P0776 Pressure Control Solenoid 'B' Stuck Off
Possible Causes
•ATF level is low, or the fluid is burnt or contaminated
•Inspect for a stuck or sticking trim valve in control valve body
•Control valve solenoid is damaged and needs replacement
•Transmission requires an overhaul
Engine running; system voltage from 9-18v, Output speed over 125 rpm, Turbine speed over 60 rpm, and the TCM detected an incorrect oncoming ratio, during a forward range shift, for an accumulated number of occurrences. The Trim B solenoid is used to control oncoming, off-going, and holding pressure to any one of the five clutches. This solenoid is called a pressure proportional to current (PPC) solenoid since the output hydraulic pressure supplied by this solenoid is proportional to the controlled current command.