It's easy, use a laser level, a 20 ton bottle jack, torch, and 2 lengths of chain. Weld the chain to the bottom of the bottle jack and to the ends of the axle tubes, preload the axle then use a rose bud to heat the bent area. Use a laser level to confirm the straightness-then go 1/4" past straight for the spring back of the tube.
Things get a little trickier when the tubes are bent and twisted but it's possible to straighten that too, at that point it's all where you weld the chain on the axle tube.
It takes some time, but I fixed a 9" housing that was bent backward at the housing and twisted 1/3 the way from the axle flanges.
I second the alignment bar especially if it's a rear axle and if it's going to be street driven.