In pretty much all cases I agree, but having buried my LC to the frame in Utah mud...There is no reason off road driving specifically would cause a crank seal to leak and more than any other type of driving would cause a crank seal to leak.
Something else is afoot.
I had to replace the front axle and pinion seals. Harder to get mud in your crank seal but it will do a number if it gets in there.
My two cents, after all we are on IH8MUD.