Weak 2nd Gear in 4HI

2000 Blazer LS 4x4. In Low 1, Low 2, Low 3 and Drive the truck moves fine in 2HI. I went to pull my snowmobile trailer out of the garage on the weekend and there was about 6 inches of snow in the driveway so I had it in 4HI. I went to shift it to low gear while still in 4HI and didn't pay attention and shifted into 2 instead and the truck wouldn't move until I gave it 3/4 throttle. Dropped it into low and it was fine. Went into 3 and D and it was also fine. Back to 2, still wouldn't move until I almost had it to the floor. Shift out of 4HI to 2HI and 2nd gear was fine (rear tires were spinning with minimum throttle) Whats up?