One wheel at a time could be accurate if you had 3 other "dummy" scales to keep the truck level. At least that's as far as my theroretical engineering has taken me. It could be accurate for your own body weight too if you could keep your body in the exact same position for both measurements.
Speski OffRoad LTD, Cargo Solutions for Toyota 4x4's http://www.SpeskiOffRoad.com
"80 series, with a fridge"
1976 FJ40, Project with little progress.
2004 Starcraft 10RT