I'm just assuming that the axles are mounted as far back on the frame as possible.
The rotation point on the boom is very far back on that hoist.Additionally the boom seems pretty long for how short that truck is. I don't have any first-hand experience with swap-loaders, but it seems to me that the geometry of the hoist is just weird. Is the boom pin on the hoist at least above or preferably forward of the center point between the axles? assuming that your truck is within their specs, I'm wondering if the hoist on your truck was built for a long Cab-Axle (grunion) and ended up being cut down (from the back) to fit your frame.
I've picked 10 ton with a single axle hook lift (f750) with no problems, definitely felt it unload a bit, but nothing like that. There is definitely something screwy with that.
I'm not going to get out paper tonight and start doing force diagrams, but I'd demand the manufacture gets an engineer involved to figure this out. It's clear that this isn't just a matter of just a funny situation but a clear error at some point. At some point in the design process engineers must of spent a lot of time figuring out how to avoid having this happen.
UVM SBA 2012