The dk45 that cracked had the full subframe. The guy just doesn't know what to do. He had it welded once, didn't hold. Cast iron just doesn't always take to welding.
Welding cast iron is tough. To do it properly, one needs to disassemble the components of the bell housing (or transmission case, if that's what it is). The casting should be pre-heated to about 1500 degrees and the rod to be used is formulated for cast iron.
My guess is the original welding was done on the tractor and no pre-heat.
More info:
Guidelines for Welding Cast Iron
The Deere 448 backhoes use two plates mounted to the transmission housing and ROPs mounts. Once the 3PH draft links are removed, the 'hoe is lifted with the rockshaft lift links, two pins (each about 1.5" in diameter) are installed through the 'hoe frame and tractor mounted plates. The lower 'hoe frame sets in two slots in the tractor mounted plates. After all that, the lift links are lowered and the backhoe hydraulic hoses are plugged into the tractor's hydraulics.
It's a pretty robust and the stresses seem to be well distributed.
The manual doesn't specify it, but the illustrations indicate the stabilizers are to stabilize, not lift the tractor. I've only used my backhoe once, and that was just to try it out (I didn't buy the tractor for the backhoe...backhoe was just part of the deal) and all four tires were on the ground.
As far as the DK45 (topic of this thread), it's entirely possible the backhoe usage had nothing to do with the casting failure. Castings tend to have other flaws common to the casting process...and these flaws can (and have) lead to cracks.