allison trans problem?

[HEATHN]

i have a 2004 2500hd duramax w/ allison trans. 26,000 miles on truck. i did a

trans flush at about 21,000 miles. my problem occurs at full throttle shifts. the

engine hits rev limit and the transmission hanges in the lower gear for 3-4 sec.

before shifts. under normal driving conditions it does not occur. i took it to the

dealer and they could not figure out the problem. they tried to blame my tires

(295/75/16) and the aftermarket exhaust. they say it is causing the computer to

be trown off. they want me to remove the items and return the truck to stock

equip. and bring it back to ckeck. i think they are digging for excusses for the

problem. the computer threw a code for a fuel pressure problem but they ck'd it

and said it was functioning properly.it almost feels like the trans is going into limp mode. has anyone experienced this problem and if

so what was the outcome. thanks for any imput.