Just to add an additional thought to this thread, you also need to be aware of your cars corner weights. I know that many racers do run there cars over scales and know what their wheel weights are but my guess is the majority don't. If you have a car that has a short wheel base, stiff frame, especially in torsion, a wide rear axle (I think I am describing a roadster!) then cross weight can have a big affect on if your car will go straight regardless of the open or locked diff. The weight that the rear wheels carry is pretty constant, except during heavy acceleration, so any weight that is taken off of one rear wheel by either static cross jacking or dynamic cross jacking will be transferred to the other wheel, which of course make that wheel drive more and because the rear end is wide it will generate a greater moment about the cars CG and try to drive it in one direction. High spring rates or solid mounting of the axle make it even more sensitive to dynamic jacking. If you happen to be thinking about building a car with completely solid suspension both ends you had better make at least one end adjustable or you could build in cross weight that you cannot change.
Just an additional thought.
Rex