An update from my previous post for anyone still working on this.

Did not have an opportunity to run the truck last week, no deliveries came up.

Used all the "old" diesel, installed a new fuel filter, drove around town a bit last week. Delivered a moderate load today, certainly not qualifying as "heavy". Headed to Austin on Hwy 71. 18 miles from the farm (about 60 on the fuel filter) and code thrown. A/C on, Cruise off, tow haul off, just driving normally, trying to keep above 70 mph, 81 degrees out, wind left to right 15-20 mph, moderate incline (nothing near 6%).

Modified my driving habits mid trip. I would accelerate down hill and refrain from depressing the accelerator pedal when headed up hill. Even pulled my foot out of the pedal on some of the steeper inclines.

Got to a section of 71 where there are some traffic signals. Used this opportunity for some full throttle acceleration. Couldn't get a code no matter how hard I mashed the GO! pedal. This is interesting to me. I'm getting up in the rpm range, I'm figuring my injectors are staying open a little longer, turbo is winding up, boost PSI is pretty up there, this motor is giving it everything it has, demanding higher fuel rail PSI. Why no code, especially with the injectors staying open longer dumping more fuel? Sure the CP3 is spinning faster, probably has a pretty good vacuum on the suction line (any hose pinching problems should be pretty obvious).

It got me thinking about it, this code is throwing under 2,000 rpm in high speed and loaded conditions in high gear.

Got to the delivery site without much opportunity for further testing. Oh well. Continued to ponder on the drive home from Austin and took the opportunity to test a little more.

I've narrowed it down to these conditions:
(1) Transmission in drive
(2) Any speed with the transmission in 6th gear (for me 52 mph or higher)
(3) A slight depressing of the go pedal while going up hill (even some not very steep) (pushing it to the floor makes the transmission down shift)
(4) A trailer connected (even an empty one)

With these conditions, this code throws every time.

So I popped it into manual (M on the dash) and selected 6 as my range to see what happens. Couldn't get the code to throw to save my life. I even turned around and went back to the last hill the code popped on. Still couldn't get it to do it.

So for now, I'm driving in Manual.

Further thoughts: Why does this code pop only at highway speeds with slight desired acceleration? Something to do with the transmission and engine communication? Anybody know how many sensors are in an Allison transmission? Why does the transmission, under full throttle acceleration, shift from 4th to 5th at 2700? Why is there a 6-700 rpm drop going to 5th? My foot is still in the pedal, I wanna stay up in the power band, not drop below it. Why, when the code throws, am at 92% (or more) calculated load? Where is this load measured or how is it figured? Why does my truck think it is so loaded with an empty trailer weight of 3,000? Is my transmission causing the increase in resistance? We've religiously maintained the axle bearings on the truck and trailer, they spin very easy by hand.

I'm beginning to think my problem isn't in the fuel system, its in the transmission. Something is telling my fuel system to go to max PSI with a slight depression of the accelerator pedal, WITHOUT down shifting the transmission, allowing more RPMs, boost PSI, etc.

Anyone know anything about an Allison 1000 transmission? I found this video:
http://www.youtube.com/watch?v=8xFwoTCInY8

(According to this video, Allison 1000 gear ratios are 3.10, 1.81, 1.41, 1, .71, .61, hence the big drop from 4th to 5th and the only slight drop from 5th to 6th.)

Do I have a failing torque converter? A faulty pressure sensor in the transmission somewhere? A leaking valve body? Is there anywhere I can plug my scanner into this thing to find out? (No there isn't, unless your trans is post 2009, when "prognostics" was added).

I remember older transmissions had a "dump" valve that was mechanically operated by the opening of the throttle. In higher gears, this valve would cause enough loss of fluid psi to down shift the transmission allowing the torque converter to spin faster and thus re-raise fluid psi while accelerating the vehicle. How is that accomplished these days with all this wiring harness connected to this transmission?