The same thing happened to my truck...twice.  Turned out that the first time my gears were fried.  Had such a loud hum on the highway I could barely hear the radio...both times the noise started at about 45-50 and just got louder the faster I drove.  The second time my ring bolts came loose...once again causing my gears to get fried (same noise that time too).  Did you do the break in on the gears AND drain and refill the fluids??  I think that was my problem the first time (and not having a CV rear driveline)...I really think that the guy before me put the gears in and just drove on the highway; not paying attention to the temp of the housing or drain the fluid.  Pull your third and check it out...might save your gears from turning to crap...if that is the problem.  Here is the link to what happened the 2nd time http://board.marlincrawler.com/index.php?topic=35841.0  don't let your gears get this bad...pull your third and check it out!