03 Tacoma 4WD, Noise in Front End

2003 4WD Tacoma with 5000 miles has a low frequency growl (sounds like when you run over a rumble strip) at 25-30 mph. I only hear (or feel) it for a few seconds every 3rd or 4th wheel revolution, and only when cold (below 40 degrees or so). After about a mile of driving the problem disappears.

I've contacted multiple Toyota dealers, but none claim to have heard of this problem. One dealer told me that its normal and that the fluid in the front differential needs a little time to distribute itself among all the moving parts.

I've tried replacing the front differential fluid with a synthetic variety because of its ability to flow better when cold, but the problem persists.

On a related note, can I easily convert this vehicle (ADD differential) to use manual locking front hubs? I suspect that this is more than a simple bolt-on replacement, but can't really find specifics anywhere. What exactly is required to change this out to manual locking hubs?

Thanks for the help...