i have to agree that looking at it retrospectively, i feel i've wasted my money on the venture diff. Maybe in the really long run i'll get a little less engine wear doing 500rpm less for the same speed, but even that will that a long long time to actually see a saving of any kind.
Changing the 5th gear for the venture overdrive is definitely a better option as i feel that this would really help by dropping the rpm a lot more for highway cruising, but again the high cost of splitting the cases would take a hell of a lot of miles @ $4/gal to even out.. It's only worth doing if dropping the motor is a must for other repairs.