Diesel engines tend to run hot at full load, I know the diesel in my VW
will tend to overheat at wide open throttle at RPMs of 3500-4000.  I did
a few things to help it out, one was an manual control to turn on the
stock electirc cooling fan, then I added an overdrive water pump pulley
(smaller WP pulley to make it spin faster and pump more water), upgraded
the radiator (in my case 2-row core replaced with a 3-row unit) and also
turned up the max-load fueling and advanced the injection pump timing
for more torque. So far these all seemed to help control the
temperatures to a great degree, may also be adding an oil cooler to help
shed the excess engine heat.  

The main reason diesels run hot at higher RPMs and loads is that they
get less efficient at the higher RPMs and you end up pumping more heat
into the engine with the faster compression strokes and the increased
fuel.  

Before I did this, I used the technique of driving by the temperature
gauge up hills.  At the bottom of the long hill, I would turn on the
radiator fan, then drive hard until I hit about 1 mark below my maximum
"comfortable" operating temp and then would slowly back off the throttle
as the temp. gauge inched up to my limit and then I would maintain the
temp. reading with throttle and gear selection.  Often I would run into
grades where I could physically drive up faster, but at the risk of
overheating the engine.

I putted a auxiliary electric fan (with a manual switch under the
steering wheel) in front of the existing radiator because the authentic
cooling fan is mechanically connected to the motor and turns with the
speed of that motor.
The block has thoroughly been flushed.

It's an 4Runner. In Europe we don't know the type 'Surf'.
Chris