As far as the the truck with 4.11, the tall gearing will do that, the 2.90 has short gearing, so it causes it to take off, can't explain the middle one being a dog, maybe it doesn't have as powerful an engine as the others? Tall gearing causes slow acceleration and higher fuel economy. I know that most W bodies with the standard geared trannies go into 2nd at around 40 mph, while mine shifts at 20-25. As you probably know, you get the most power (torque maybe? sorry if I'm using the term incorrectly) in first gear (well, other than reverse, which is really tall relatively on any car, and teh reason why its the easiest way to get out of snow nice and slowly) so that may explain why your plow can plow a mountain yet still only shifts at 50+ mph.
2004 Chevy Impala "9C1"
3.8 Liters of 'Merican Pushrod Power
56,426 miles as of 7/1/2012