4x4 is 4x4 no matter what combination of open diffs, lockers, posi's, or spools. You may only see 2 wheels spinning, but the other two wheels are trying to drive with the exact same amount of force as the ones spinning.
True, in can be unequal "power" if we are talking horsepower. but just "power" is too broad of a term.
Horsepower is 0 if there is no rotation. So while torque is being applied to try to turn the wheel, if traction is great enough that it dont spin or roll, then yes, technically, there is no horsepower.
As to why the tractor dont buck like a truck, I think it is a combination of what most others posted. First, the fronts do rotate a tad faster, so that helps. But with everything being ridgid and no suspension, they just slip and you dont know.
There were/is exceptions to the trucks though. Old full time 4wd trucks with a 203 t-case didnt buck, cause they had a differential gear in the t-case just like the axles. It allowed one axle/driveshaft to rotate at a different speed.