Hey,
Well i have a 2003 7.3 lariat ccab 4x4 and i have the exact issue, i had it in 4x4 also, had the front drive shaft rebuilt and balanced, that fixed that, then i also had it in 2wd, and it only comes as your gaining speed slowly or if you lightly touch the throttle i can hear the growling/vibration and its driving me nuts, at first i thought it was a wheel bearing and replaced both assemblies, 500$ later :\ my problem wasnt gone, so ive been searching for this for awhile, the rear ujoints i havent checked yet but will be doing tonight ill just drop of shaft and see how stiff they are, what brand does everyone recommend? and of coarse greaseable.