The "frame beef up kit" does not eliminate rear end separation.
For most part, all it does is separate you from some of your money, and creates a false sense of additional frame security.
Axle mounting plates sometime show up with a ripple, which is caused by the axle mounting plate not being properly welded to the frame.
If someone wants to change the tandem axles out to say 5000 pounds each, because they will be creating a trailer that will be used for a restaurant business, or the like, then the axle beef up plates would be in order.
But for normal use, NADA.