1989 Dodge Ram Troubleshooting my pick up

I own a 1989 Dodge Ram 150 4x4. It is a day cab w/ a short bed and a 4 speed tranny. I have owned the truck for about 3 months and have plans to restore it. It also is my everyday vehicle.
I was living in Colorado, when I decided to move to southern California. I planned on driving here. After about 500 miles in Nevada, I all of a sudden smelled gear oil and had a very low toned humming or grinding sound. I checked all the boxes including the tranny and they were all full.
The next morning I decided to pull the left and right inner and outer bearings and races and replaced them. I then decided the problem was fixed. Re-assembled and started heading back on my trip. About a half mile went and once again felt and heard the same noise that I had heard the night before. However this time, in was intermittent. It would come and go. Im in California now and will be going back to Las Vegas in a couple of days to try and figure out whether I should replace parts in the rear end or not. What do you think? Or where should I begin looking from here?