9 speed vs 8LL

I have a heavy spec dump truck with 11.1 detroit and an eaton 9 speed. I'm really nor sure of the rear gears but it will easily do 60mph at 1400rpm on the highway. My question is this: can I replace the 9 speed with an 8LL and still do 60-65mph? Are all 8LL geared the same and if I find an 8LL and swap it in place of my 9 speed, what's the difference in gearing between the 2? I found an 8LL for $2,000 that has 160,000 original miles and I hate that my 9 speed isn't low enough off-road. Is there a better option than the 8LL as an upgrade because I really don't like the long distance between each gear in the 9 speed.