I had this same question:
Jacking up MY fiver (sigh, again)
Basically, what I got out of the answers was:
First- never jack anywhere
on the axles if it's torsion axles. No one argued this point.
Then, there were 2 distinct schools of thought:
1) Don't jack on the axles. Many manufacturers recommend against it. Doing so can damage the axles. Use the frame instead. But, do put a piece of wood to spread the load a bit.
2) Don't jack on the frame. Doing so can cause damage to it. Use the axles- manufacturers recommend against it because people will try to jack in the middle and break things. Jack the axles up closest to the U-bolt hanger because if it can withstand the stresses of traveling down the road, it can handle being jacked up.
So, there you have it- 2 answers, both certain that the other side is wrong and 100% opposite to each other.
Personally, I bought a 3-ton floor jack and jacked up on the axle as close to the u-bolt as I could get. I did ultimately start to use a piece of wood between the jack and the axle to try and spread the load a little- but I haven't seen any negative effects. But of course, as they say- your mileage may vary.