I dont know whats wrong with your truck but an allison in a truck running 50 or less stop and go should leave the others all day long, and the engine braking should be excellent. I know I get a lot of crap from some on here advocating allisons for vacational work but they are proven performers. your D11 is not the same as your aset in that granite, the aset engines from 04-06 are turds unless you get em set right, the DD11 will outrun it in that regard. I would get the trans and engine calibrations checked it should run better than the manual trans trucks in town for sure. If you swap to a manual you will have to reprogram the ecm, and if you want it to run the same speed as the others change the gears, allisons are true double OD your looking at a .67 6th gear that is gonna have a rear somewhere in the 5's if I had to guess which means with the .74 in that 8LL your gonna be geared way low the allison has a 4.70:1 forst the 8LL has a 14.56:1 your looking at second gear being the equivalent to your first now, then topping out about 55 mph at 1800rpm. I say something is wrong somewhere it should run great stop and go, and with the engine brake on it should go into lockup and drop gears to keep the rpm up you should be able to stop it without much brake pedal at all. I would take it to a Mack dealer and have them check the claibration and look for updates on the engine, you can have them set the shift points where you want em and with an aset they should be around 1600-1700 to keep it in a pull, also check for boost leaks in the intake system. just me I doubt its running right if its that bad in stop and go I never could keep up with the automatics here and my Granite was turned up quite a bit, obviously I could get em after we were up to about 40 then I had better gear steps but thru town it was no contest.