I assume your asking about the std leaf spring suspension.
The reason axle mfg don't recommend jacking under the axle is is folks have no common sense and place the jack toward the center of the axle which can bend (more camber bow) it.
I've jacked loaded GN equipment trailers with 1750 lb axles on up to 10k lb axles from the bottom of the axle tube next to the U bolt....or the U bolt itself. Doing so will not crush the axle tube or bend anything as all the jack is lifting is just the weight on that end of the axle (2500-3000 lb on a 6k axle).....and not the whole side of the trailer.
I'm not a fan of any kind of single axle trailer however some of my boat trailers came with one axle. The old jack easilly slides under the axle tube/next to the U bolt and lifts one end of the axle enough to change the flat tire.
I use a old 12T truckers special that has a adapter that cradles the axle U bolt and one that cradles the axle tube.
Others do it the way they want.
"good judgment comes from experience, and a lot of that comes from bad judgment" ............ Will Rogers
'03 2500 QC Dodge/Cummins HO 3.73 6 speed manual Jacobs Westach
'97 Park Avanue 28' 5er 11200 two slides