humming/grinding in 4x4??

i have 2000 2500 4x4 with about 250,000 miles on it. the 4x4 has always worked great with no issues when needed (rarely) and i dont abuse the 4x4.
yesterday it was about -13*F (cold) and i used 4hi to the store, put it in 2hi on the highway, then put it back into 4hi when leaving the store, and it began making a humming/grinding noise so i promptly put it back in 2hi.
it was defiantly locked in to 4x4 and not slipping. it is not acting like it is a transfer case issue, or an engagement issue. the 4x4 is working like it should, it just makes a low hum of something grinding now, that speeds up with the throttle. otherwise, everything seems fine. i thought it was just something to do with the extreme cold but is still doing it today when i tested it in much warmer weather.
it only makes the noise in BOTH 4hi and 4lo, which leads me to weed out things like front wheel bearings. it dosnt sound to be coming from the left or right side specifically, making me think its not a CV joint (they also both look fine).

doing a quick search online leads me to believe it is most likely my front drive shaft. i read a few places of others with a similar issue and it happened to be a bad joint on the front drive shaft.

has anyone else had this issue who could confirm my diagnosis? are there any other ways to more specifically diagnose this to be sure? would a drive shaft (or something else) just fail like this so suddenly with no warning?

thanks for you help.