I assume by 'LOW 2wd position' you're referring to the transfer case gear selections in most 4x4 trucks. It's called a transfer case (TC) because it transfers power to the front axle AND the rear axle. It used to be very standard that any 4x4 truck TC would also have an integral 2-speed range box - a Hi gear range for most driving situations, and a Low gear range (usually 2:1 or higher) for serious off-road or pulling situations - like full grain hoppers or other heavy ag situations for farmers.
When the transfer case is in Low range in most normal 1/2 to 1 ton pickup trucks/SUVs I'm familiar with, the engine is at the RPM redline at around 30-40 mph. They're supplying the needed torque to pull a heavy load (without excessive Auto trans fluid heat & torque converter slippage, or Manual trans clutch slippage), but forget about pulling for a long distance at highway speeds - ya can't get 'em that fast.
A lot of the more modern push-button light-duty All-Wheel-Drive trucks/SUVs no longer have the 2-speed range box.
Medium duty trucks (450/550/650/Kodiak/Top-Kick, etc) are a different matter. Rear axles with internal gear range selection are available, or after-transmission overdrive/'gear splitters' are made, so you can shift from 1-Lo -> 1-Hi -> 2-Lo -> 2-Hi ...and so on. But these trucks usually aren't 4x4s.
The smallest rear axle ratio I've seen commonly available from Ford/Dodge/Chevy in towing-rated trucks is in the 3.50s, with a lower weight capacity. 3.73s and 4.10s are more common and feature higher weight capacities....but lower fuel economy.
Read the manufacturer's tow ratings for each unique body/frame/engine/transmission/axle ratio combination carefully, because there's a bunch of differences in the tow ratings.