What causes the rear main seal to leak only while pulling a heavy load on a 5.4l

The pressure inside the block builds on higher mileage vehicles, and even more so while working harder when pulling heavy loads. The pollution control system helps relieve the pressure on the block with in most cases the block vent system. One might consider checking and/or changing the PCV valve it's tubing and check for clear vent passage. If there is some wear on cylinders, rings, etc. the pollution control might not be enough to relieve the pressure building up in the block causing the pressure to relieve itself at a weak spot, 'ie' a weak rear main seal. Hope this helps.