I run an Advanced Adaptor Ranger Gear Splitter in my '85 F350. For me it was a cheaper option than a ZF swap or Gear Vendors to get overdrive into the truck. All in, it was about $1700 (2016 money), including shortening the driveshaft and new carpet. For me, it functions as a twin stick 5 speed. Start out in low range, 1,2,3,4, Up shift into 4th-Over at highway speeds. The only time I split any other gears is if I have a big trailer hanging off the bumper with a ton of weight, then it can be nice to split the big gap between 2 and 3. 3rd-Over is essentially the same ratio as 4-low with my 4.11 rear axle. The only difference I have noticed since switching from the stock 6.9 to a 7.3 Turbo is I can run in OD while towing much, much more consistently and don't need to down shift for every little hill.
Shame that Advance Adaptors has discontinued production. They are good units, bit noisy, but stout.
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