Problem with choosing the right rear axle ratio

There is no problem, the truck is simply not setup correctly for your application.

Go online and punch in your truck specs into an RPM calculator (Spicer has a pretty good one). Just play with different rear end ratios until you get the cruising RPM where you want it.

 

Are there any codes? Is the transmission actually getting into high gear?? There should be a sticker on the drivers door jam to verify the rear axle ratio and the tire size that the truck was manufactured with. 24-2500 isnt all that hard on the 6.7 as its rated speed is up around 3000rpm.

 

What size tires to you have?

 

Also need to know exactly which transmission you have to determine what gear ratio’s it has.

 

Sounds like the op has 19.5" rubber. Being as it is set up as a roll back tow truck, someone may have installed 19.5" rubber to lower the unit.
I own several units all with Allison 2500 transmissions and 3 different axle ratios.
All on 11R22.5 rubber
5.29 at 55mph is 1750rpm
5.57 55mph is 1850 rpm
6.17 55mph is 2100 rpm.
All in 5th gear. Allison 1000/2000 have nearly the same ratio in the top 2 gears..

Most allison 1000/2000 of that era only had 5spds available, Allison disabled 6th unless the manufacturer engineered for 6th, allison won't just allow 6th gear without it.

Use an online calculator too see what you have. 5th gear in the allison should be 0.71

Edit: op posted the 255/70r22.5 after I posted, which are lo pro 22.5s..
6.7 isb in medium duty is 2500rpm max