I should also point out that the Fuso FG 140 is not really a "high-capacity" 4x4 diesel truck. It is, however, a fantastic "low-capacity" 4x4 diesel truck chassis.
The F550 has a higher GVW capacity than the Fuso, and I guess would be on the low range of "high-capacity," depending on what your perspective was. If you're starting with a 1/2 ton pickup, then the Fuso looks pretty darn high-capacity.
I guess I was thinking U.S. class 5 or 6 and up when I started this thread. That may not be fair to people who are looking at this from behind the wheel of a Jeep or a Taco.
However, if you include F550s and equivalent ~19k GVW trucks, as you point out, you'd end up with a ton of listings.
Personally, I'm intrigued by the more obscure, higher GVW 4x4 and 6x6 trucks.
Note: Intrigued often equals "money pit," "lost cause," and "they haven't made parts for that in twenty years" in my world. Danger, Will Robinson.
From Wikipedia: http://en.wikipedia.org/wiki/Truck_classification
The Class 4 truck gross vehicle weight rating (GVWR) ranges from 14,001 to 16,000 pounds (6,351 to 7,257 kg). Examples of vehicles in this class include select Ford F-450 trucks, Dodge Ram 4500, and the GMC 4500.
The Class 5 truck gross vehicle weight rating (GVWR) ranges from 16,001 to 19,500 pounds (7,258 to 8,845 kg). Examples of trucks in this class include the International MXT, GMC 5500. Dodge Ram 5500, and the Ford F-550
The Class 6 truck gross vehicle weight rating (GVWR) ranges from 19,501 to 26,000 pounds (8,846 to 11,793 kg). Examples of trucks in this class include the International Durastar, GMC Topkick C6500. and the Ford F-650
Vehicles in Class 7 and above require a Class B license to operate in the United States. These include GMC C7500  Their GVWR ranges from 26,001 to 33,000 pounds (11,794 to 14,969 kg).
The Class 8 truck gross vehicle weight rating (GVWR) is anything above 33,000 pounds (14,969 kg). These include all tractor trailer trucks.