Here we have a used 6sp allison transmission assembly removed from a 2013 GMC Denali 3500hd with only 90,000 miles. This truck sustained a heavy left front suspension damage which tookout the suspension and pushed the transmission back which broke the bellhousing and rear taillight housing which you can source from your old transmission. There is no damage to the actual transmission. This should fit the 11-14 trucks with the diesel engine and maybe others. Make sure this part will work for your truck.
Removed from Vin # 1GT125E88DF152444.