Since you have tandem axles, the weight will still be distributed on both axles. As noted above, don't use the point of lift the axles, get under the shock mount togive yourself some extra protection. I would buy several 1x8's and 2x8's to help level the easy way when driving up on them.
I think you will need to block both tires to be effective, not just one. You will only flex your leaf springs when jacking up.
Bob and Better Half
2007 F-250 PSD, SW, CC, LB, 2WD
2006 TF32SS Cross Terrain Toy Hauler (sold)
Reese 16k slider