If tire to fender clearance is not an issue to get a back tire off, I just throw the jack under the diff near one of the shock mounts, not the center. If your leaf springs are bagged out, doing this may lift the tire too far into the body to get the tire off. If you put the jack on the frame, you might have to lift the frame quite a bit before the tire gets off the ground. Worse if you have sagged out leaf springs. For the front I just throw it under one of the frame rails because the front is lower and doesn't sag as much. If it's my jeep or truck I'll put it under the control arm. If I'm just changing tires, I'll leave it on the jack. If working on anything, lower it down onto a jackstand.