There a few extra steps I take when I install the an axle on leaf springs.

I make sure that the u-bolts are perpendicular to the springs. If you are not careful they can sometimes get some angle to them. As the spring works the u-blots move and get loose.

I start the assembly by getting the nuts finger tight while I jiggle the u-bolt so it will seat itself. I work a cross pattern and tighten each leg just a little bit at a time.

I finish tightening the nuts with the vehicle on the ground. This pushes the axle up against the bottom of the springs.

If the axle is above the springs I let it hang while tightening the nuts.

Rusty