Well I had two 15's and yeah you can split all the low range gears in em, they are a 10 speed with an added deep reduction range I was trying to simplify my answer, I get told I tend to get too technical for some. I had two 15's over the years they are great transmissions for heavy haul or on/off road use, I actually have always shifted my 8LL's like a 15 when needed, they will split all the low range gears to deep reduction also they are just not made to do so, the 15 has heavier components for all the low range gears to handle the added torque the 8LL just has them in L so you can kill the other low range gears if you put too much load on em in deep reduction. Actually you can put an 18 spd shifter on any eaton trans with an aux box and split every gear in the trans, they just arent made to do so and youll kill em doing it, I have put an 18 spd shifter on both a 13 and an 8LL just to try it and yep it works the block to keep you from splitting the gears in the wrong range in a 8ll,13,15 is in the shifter not the trans. Actually all Eaton transmissions with auxilliary range split, 8LL, 9LL, 13,15,18 all have a deep reduction back box on them, the OD is in the main box so when the splitter is in high you are direct in the back box in low you are in deep reduction, so in a 13 speed in 8th gear low you are in a deep reduction gear and 8th gear high is 1.1, 9th gear high is .74od and 9th low is .86 because of the deep reduction split of the OD main gear. Just my two cents but for that truck if your just tired of the 9 spd go find a 15 somewhere and swap it out, youll be happy with it there is always a good gear for off highway with them and a used RTO14715 or 14915 is a lot cheaper than an 18.