What David said but I use one heavy piece of angle iron as a V block, as long as you can fit between the wheels. Clamp the axle to the angle on each side of the joint. I have Vice grips that have chain which you can wrap around the angle and axle to clamp it into the V. Chamfer the ends and start welding. Once you have some weld built up in the bottom of the joint put the axle ends up on blocks so you and turn it and find the low spot, weld there to draw it straight. Just keep at it till you have enough weld and running straight.
Greg