86K mile 03 2WD DD/ Allison Auto has been hesitating/ bogging down at redline between 3-4 gear. Thought trans was slipping until I watched the tach and saw the needle just sit at 3K RPM after 2 seconds it upshifts.

Think of leaving the tranny in 2nd gear and riding around at redline for a few seconds then upshifting. Trans fluid is good, was changed 16K miles ago and is still red/ not brown.

Any ideas?

De fueling at or between shifts??...computor management software issue..

I borrowed my sisters 4 month old '02-'03 DD/ Allison dually to pick up my boat. It had trouble just pulling my empty trailer. And I was going from IN to OK/TX. It had trouble staying at highway speeds. In town it was great. It just had no power what so ever over 40mph. By accident, I found out that the CC would make the truck run like a raped ape, so I drove it by using the CC control at anything over 40 mph.... for the 20 hr trip. We were passing cars and trucks like no tommorrow, going up mountains and were picking up speed, as long as we used the accelerate button on the CC. So the power was there.
Sis took the truck in after I returned and they found that the foot throttle assembly was bad, which explained why the CC would work fine. They mentioned they were seeing alot of that problem.

For chits and grins, see if using your CC acceleration button makes any difference.

Have you changed the fuel filter lately?

About 4 months ago. Done every 10-12K miles to try and avoid any clogged injector issues down the road.

air filter too?

New KN filter at the same time as the fuel filter.

Defueling....what codes is it giving....you need a mechanic with a tech 2 and a candi module....when you read the codes out....you can then re program....the transmission is a self learner....codes P0700???...it has to be giving you a code of some sort....I just cleared mine after some work.....and have to teach it all over again....do you have a tuner programmer of some sort????....it doesn't matter anyway....the factory trans will defuel the motor for shifts....just not a much as tuned....these things are totally bad azz combos...Byrdmans Hot Rod Diesel Injection Shop lmao

I would try changing the fuel filter again even though it was changed not that long ago. I had a simular issue and changed the filter and it was all good .you can get some bad fuel at any time. fueled up once by chicago and the stuff had so much antifreeze in it I barly made it home ended up draining the whole tank to get it to run again

try a spin on trans filter,and have it ck'd for a bad nuetral safety switch,, my 06' will pull a 42' cig like its not back there