Drive axle alignment?
Seems I had this problem once, and that made a difference.

 

If you have eliminated the other suggestions, you might check your fuel lines. Your stated mileage is is minimal for this, but entirely possible. What happens is the interior of the fuel lines start to collapse, cutting off your fuel. This is magnified under a pull.

 

A torque sensitive vibration is usually coming from the driveline.

Check for loose yokes at the output shaft of the trans, and at the input shafts of both diffs and the output shaft at the rear of the power divider on the front diff. Also check for worn slip joints on each driveshaft, and worn U joints.

I just thought of something else:

Check the harmonic balancer at the front of the crankshaft too, a defective or worn out balancer will cause vibration on a hard pull, and will eventually result in a broken crankshaft if you continue to run it that way.

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how about the crankshaft vibration damper? pittsburg power says they are only good till 500k mi. if your mileage is higher than that, maybe you should have the damper checked because if it doesn't do its job properly your vibrations will increase. pittsburgh power sells new ones for cat engines for around 300 bucks.

 

Could be many things:

Worn input shaft
Clutch worn/out of balance
Worn engine mounts
Sloppy spring bushings
Tires out of round
Drums out of balance
Exhaust leak under cab
and what already was said...