4200' Elevation could have a bearing on this. You lose about 1" of manifold pressure for every 1000' of altitude above mean sea level. So instead of approximately 30" Hg, manifold pressure, you're getting around 26". This has a direct relationship with how much power the engine puts out.
Without dragging out the formulas, I'd say at full throttle you're getting no more than 86% of your engine's rated power. And you would get that much only if fuel/air mixture ratio had been optimized for that altitude. How they tune a diesel for high altitude operation, I'm not sure but I'd suspect it would have something to do with the injectors. A fuel/air mixture ratio appropriate to sea level operation would be fuel rich at 4200 ft. and could cause smoking.
This probably isn't the single cause of the problem, but it's a contributor. I think Ramon could probably sell the tractor to a lowlander with both a good explanation of why he's selling and a clear conscience /forums/images/graemlins/smile.gif.
Bob