i posted this tin the drivetrain forum a few days ago but got no response. maby some one here can help?

ok so my truck has always had a howl to it especialy between 35 and 45 mph. it was always constant with vehicle speed. from a low tone to a mid tone. always the same tone at it respected speed regardless of the gear or engine rpm. so i always assumed it was the rear axle. but now that it has been sub-zero outside its begun to do somthing strange, and kind of hard to discribe. accelerating from a stop 1-2 shif is fine 2-3 shift is fine but at 35 to 45. after 2-3 shift and light (crusing) throttle the tone drops at almost regular intervals. (if at 45 sound drops to the tone of 35) it will cycle like this until you let off the trottle or get in to the trottle or the trans is fully warm ( after about 20 miles highway speed). i know the fluid lvl is good. it is a little brown but dosent smell burnt and has no metalic residue on the dipstick. it does not slip at all but a slow 2-1 shift uphill takes a bit of trottle. well come to think of it, it seems to like to shift in to second quite early under light loads... is this a vaccum issue or are there other thing i should be looking for. oh and i almost forgot. its an 85 f150 351W c-6 9"rear with 3.0:1 gears. if that helps at all.

 

Lots of things don't work the same in sub-zero temps as they otherwise do, this isn't
surprising. Is it OK when it warms up to normal operating temperature?

 

Reading your post I would still think the problem is the rear diff.
If the fluid is brown I would drain the torque, remove the pan, clean, replace the filter and refill with Dexron 3.