Adding to that...
In a car, there's also the issue of sprung and un-sprung. A car is not a fixed position weight. The unsprung suspension is relatively fixed but the body is a movable weight. As you accelerate, the springs allow the body to move up in the front and down in the back effectively increasing the weight on the rear of the car and decreasing the weight on the front. While the front tires may lift slightly as you described, the amount is usually minimal. More of the traction reduction is generally attributed to the shifting of the body upward in front and downward in back
In a FWD car, stiffer rear springs can help with this issue. (Actually, they help contain the shift in RWD too, but it's not an unwanted effect there). A stiffer rear sway bar will help with the issue that one wheel tends to come up if you accelerating but not going in a straight line (but it will not decrease the overall effect).