4.0 4wd

3 months ago i replaced the left front hub/bearing assembly. 1st. howling ended.
Started howling again a week ago so i rechecked left again and replaced the right side but still howling.
Today i pulled the rear driveshaft and checked the u-joints and they are like new.
The joints in the front drive shaft feel good and tight but I wont be able to pull it out and thouroughly check until tomorrow.
Also the real axle bearings feel tight and smooth. So the questions are:

Where to check next?
Can I isolate by removing 1 driveshaft at a time and then drive? Or, will this damage anything?
Do the front axle 1/2 shafts always spin when the wheels are turning and could one of these be the coulprit?

Trying to find the noise is getting frustrating as it is getting louder quick and I want to fix it before something breaks.

Oh yes.....the howling pitch changes directly with speed regardless of load/ no load, acceleration/ deacceleration but does stop when I go over a bump and weight is reduced.