Correct - "whilst" is English:

Main Entry: whilst
Pronunciation: 'hwI(&)lst, 'wI(&)lst
Function: conjunction
Etymology: Middle English whilest, alteration of whiles
Date: 14th century
chiefly British : WHILE


Cheers,
Pete.

p.s. Harvey, not sure what you meant by "transfer valve is still going to be a problem on the center diff model." ? An ongoing discussion I'm not aware if maybe?