'93 Dodge Caravan Humming Noise

My '93 Dodge Caravan 3.0 lately started making a humming noise accompanied with a slight vibration perceptible after 40 mph at the steering wheel, both disappearing below 40 mph.
The sound seems to come from the passenger side axle. As soon as this noise came on, the inner side of the front tires started to show signs of wear, so I took it to the shop to check the alignment that was done a couple months earlier, the computer showed that the alignment was within the manufacturer's specifications. So, they inspected and re-torqued some bolts, rotated tires etc. In hope that this would solve the problem.
Now, the rotated tires are also showing the same wear pattern.
I'm sure the humming sound, vibration and tires wear are related because all appeared about the same time.
Any ideas or suggestions? Thank you.