Before you try anything else: DO NOT LET THE JACK TOUCH THE AXLE TUBES!
You will be paying for another alignment on a bent axle.
Under the "spring perch", the square plate where the axle is bolted to the springs, is OK. Or, on the frame using blocking.
I carry about six pieces of 4x4 fence post a foot long in the truck bed in an old laundry bag. Useful for all kinds of things including under the bottle jack (2 wide) to get it up where it has enough reach.
Get good chocks and use them first and last. First on! Last off!