I always plow in 4 hi,unless its real heavy or im in a tight spot on steep hills.The extra shifting the auto does in 4 lo makes more heat than the added load of 4 hi most of the time,I tried it both ways with my trans temp guage,the temp was within 5 degrees in 4 hi.Another thing on gm;s the governor gear is real easy to break off when using low range,since the output shaft is spinning so fast and comes to an abrupt stop quickly,i have seen several stripped gears on the governor s,then the truck has only 1st and reverse.My trucks all have the TH 700R4 or the 4L60E,which have a low 3.06 to 1 1st gear,so with 3.73 rears this is plenty of multiplication to snow moving.My old Suburban has a TH350,2.52 1 st gear,and 3.08 gears with 31" tires,I used low range in that truck all the time,since it would do about 55 mph in 1 st gear,it was way overgeared for plowing.