Your 35 inch tires would result in substantially lower rpm's than a stock 450. I believe that our 245/75R17E's are 31.6 inches in diameter.
Has info on engine rpm versus various gears and tires.
The chat i refer to is for final transmission drive of 1:1, so it will need adjustment or maybe they now have a chart for overdrive transmissions.
You can trick the chart by using Scott's 1,850 rpms at 60 (say the 1,854 in the chart) for 32 inch tires. This is a non overdrive equivalent of 3.21 gears. Put 35 inch tires on Scott's or my stock truck and rpm's at 60 will drop to 1,695.
Are you using your aftermarket gps to calculate your reported speed, or did you have ford reset computer for your tire size? If not recalibrated, the speedometer will report a much lower speed than you are actually going.
If i recall, the final drive ratio choices on the 250/350 are offset by the mandatory swap in tire sizes for the respective ratio. You of course added the 35's to the wrong ratio for what you want to do.
Depending upon how heavy, how often and where you tow determines the ratio you need. What works on flatland with a 12k load won't work very well in the mountains, or for a tall wide front 5er on flatland. The cost of changing gears versus new tires and rims is probably a tossup, but either one will get you into the rpm range you need. Just make sure the smaller tires can support your actual load if you think about going that way.
I would think that locking the tranny on long runs is not good for its longevity.
Wowwwa that's alil steep for me 2300 RPM maybe I'll be keeping the 331s and just keep her in 5th haha just the answer I was needing to keep 2k in my pocket