I have some bad experiences on that generation of artic when they first came out. Check you steering and hydraulic schematics for the steering and box hydraulics. The ones I was taking care of were older but there was a cross connect between the box and the steering system that isolated with a ball resolver in the steering valve. We had the box lift cylinders fail internally on a set screw that held the pistons to the cylinder rods. It backed out and started shaving the insides of the barrels. The junk hit that ball resolver and dumped the steering pressures back to the tank. We had one truck returning to the pit after dumping a load and it suddenly steered to the right putting it over a 40' lift in the haul road. When we checked the other two trucks, we found the same problem in process of failing.
Check your return filter first, then with a load in the box raise it about a quarter of the way and then stop. If there is a cylinder problem, the box will drift down.