Whats the temp been there? I would also check to make sure the breather they are talking about in the TSB is not restricted in another way.
Crankshaft Rear Oil Seal Leaks During Extreme Cold Weather Operation (-29 Degrees C/-20 Degrees F) (Modify Oil Separator) #03-06-01-008 - (Feb 27, 2003)
</TABLE>Crankshaft Rear Oil Seal Leaks During Extreme Cold Weather Operation (-29°C/-20°F) (Modify Oil Separator)
2002-2003 Chevrolet and GMC C/K Models
2003 Chevrolet and GMC C4500/5500 Models
with 6.6L Duramax™ Diesel Engine (VIN 1 - RPO LB7)
Built Prior To January 31, 2003
<A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=1252276&pubid=207&evc=sm#ss1-1252276">Condition
Some customers may comment on crankshaft rear oil seal leaks during extreme cold weather operation. Typically -29°C (-20°F) and below.
<A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=1252276&pubid=207&evc=sm#ss2-1252276">Cause
A restricted positive crankcase ventilation (PCV) system may be caused by frozen condensation, ice, or snow that has accumulated in the vent tube at the bottom of the generator bracket. In extreme cold weather, the vent tube may not be subjected to enough heat from the engine compartment to melt the ice or snow. Excessive crankcase pressure may develop and cause premature oil seal failures. While the oil leak may occur anywhere on the engine, the crankshaft rear oil seal is the most susceptible due to the larger surface area in which crankcase pressures can act upon. The excessive crankcase pressure may not be apparent in the shop as the vehicle warms up and the blockage in the vent tube thaws.
(http://service.gm.com/servlets/BlobShtml?ShtmlFile=1252276&pubid=207&evc=sm#ss5-1252276)