We jacked up a lot of them that way for many years without a problem. If you're concerned about the possibility of axle damage, just jack up one side at a time by using one of the spring mounts on the frame. Straight axles are built rugged enough to handle bumps at road speed, which will create a lot more stress than using a jack on a stationary vehicle. These things aren't toys- - - -they used to WORK to earn their keep!
BTW- - - -that "2 ton" jack is a toy- - - -don't put any part of yourself under the truck until it's supported firmly by jack stands. Those little bitty floor jacks are deathtraps! Here's one I'd consider to be a minimum to safely lift a stovebolt truck, and then only on a level, smooth concrete floor. The professional-type jacks sell for upwards of $500.00 for a reason.
http://www.northerntool.com/shop/tools/product_200303997_200303997Jerry
