I have a '97 F800 with a 5.9 Cummins with 4.11 rear end, and it pretty weak for towing. I did a search and didn't come up with any results, but I am wondering if anyone has change the rear end on a F800 to a lower speed.

 

What HP level is your 5.9 set at?
Have you thought about a "(Fuel plate slide) tune up"

 

I am not sure what the HP is, but I have already had the fuel plate changed. It has helped, but the truck is still pretty weak.

 

I have a rockwell 2 speed unit out of an 87 CL7000 truck. 4.88:1 hi / 6.68:1 lo

give me a call if you're interested.. 2 5 3 2 6 1 6 3 2 4 my name's Chris. Parts in Washington state. Shipping may be too expensive.