Since it's a CV joint they're testing, 300 RPM could be upwards of 30 MPH depending on the outer diameter of the tire. I doubt a 25 ton mining vehicle gets going much faster than that.
750 lb ft of torque sounds like a pretty good load to me.
--Background math to show work: 36" diameter tire has 9.5 ft circumference (3 ft * pi). At 300 RPM it travels 2850 feet per minute, or about 32.4 MPH--