Cheapest: tall rear tires
Most adaptable to wide range or driving: more gears, overdrive
Least amount of adaptation: Gear Vendors OD unit.
Every option I've seen for OD - auto or stick - ends up at least $3000 by the time you're done, plus some require cutting the floor or t-bar crossmember.
Personally, I'm not a fan of doing that.
The more I look at the available options, the better I like the GV. Plus the one in my Dually since 1994 has been bulletproof.