True to some degree. The main issue in regards to an axle tube bending and/ or cracking often times happens where the tube enters the differential housing. It happens more often on the passenger side ( obviously due to the longer tube), but it can also happen on the drivers side. A tube can indeed bend, but again... it generally happens at the housing.
I recommend reenforcing the axle as much as possible so sleeving isn't a bad idea, but I feel that if you are going to sleeve an axle ( inner, outer, or both) that you also at the very least also do a truss of some kind in order to completely strengthen the entire axle assembly. Also... not a bad idea to get rid of the crappy-ass OEM brackets and go with a more heavy-duty set of brackets for all the attached components... control arms, spring perches, etc.
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