I have lifted lots of vehicles using the pumpkin as a jacking point and never had a problem; prolly because THAT DAY I was lucky. The jack pad could catch on the read axle cover (if present, not all axles have them) as the load came down and bend it beyond use. Some covers are thicker (on older vehicles?) and would survive just fine.
There is a good fix for this; you could make a cross beam that would span the distance(s) between the axle tube/pumpkin joint to the shock mount area. The cross beam would be placed where the jack pad on your floor jack is located. There would be a "U" shaped rest welded in place on each end to cradle the axle OR you could go to Harbor Freight and get their floor jack that accepts their cross beam made for this purpose. Some of these item numbers are no longer available, I checked the various lengths of the jacks and I picked the 2 ton that had the longest handle and longest jack #68050. This jack has the longest lifting assembly and the longest handle of the jacks listed. I'm pretty sure it'll reach the rear axle without a lot of hassle AND it's 2 tons so as long as you're not lifting the whole truck you're golden!
http://www.harborfreight.com/steel-floor-jack-cross-beam-60762.html
Note the compatibility list for suitable jacks- To be used only with the following: Pittsburgh? item 60678, 60688, 60706, 68048, 68049, 68050, 68056, or 69227.