If your axles are mounted above your springs, the flat plate below the spring through which the axle u-bolts pass is what I use as a jacking point. That's where the loading from the axle is transferred to the spring, so it's designed to carry that weight. I use a wood block between the spring plate and jack head to cushion and spread the weight. By jacking there, any vertical motion is applied directly to the wheel - you don't have to jack through all the suspension travel as you would by jacking on the frame.
Having said that, NEVER jack directly on an axle tube as they are so thin that you could crush or bend them.
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