1998 safari droning at highway speed

[wagfcc]

The droning is worse at regular highway speed and is not as bad if you go under about 95 Km per hour.
This van has 230,000 km on it. This droning started at about 100,000 and has slowly gotten worse. At about 160,000, we replaced the driveshaft going from the transfer case to the front differential, it was showing signs of wear. It did not fix the problem. Both u-joints on the rear drive line have been replaced recently.
Other suggestions involved tires, making sure to put all 4 on new, at the same time. We've also done that, no change.
Anyone have any thoughts??

Thanks,
Don