When offroading 4x4 should I use D or 321?
I've always used D. But all I do is very mild trail rides. Most of the time I'm in 2wd mode and only shift into 4H or 4Lo for grades and through mud and water or while driving on very loose dirt. I've never used 4wd mode with 1/2/3.
You didn't mention the grade, underfoot conditions, tires or gears. All of those make a big difference in the gear selection. I personally would use 2nd gear 4 lo in nasty underfoot conditions w/ 3.73 gears and stock tires. If it's just something where the rear tires keep slipping but isn't bad i'd use 4 hi and let the trans do the work. Now if there are grades you'll adjust the gear to allow for more engine breaking and less foot braking. I like using 4 lo for one reason, in very thick underfoot conditions it lessens the load on the transmission and makes it easy to clean tires. If you feel the engine lugging then it's time for 4 lo or a lower gear.


