Help with automatic hubs

I recently replaced the brakes on my 94 explorer. When I did that I found out that I needed to replace the rotor on the passengers side. Everything was fine until my wife noticed a popping sound coming from that side. When I pulled the wheel off I found that the hub must have slipped, because it was not seated firmly(when I put it on I made sure the hub was seated tightly with no spring to it). I've checked it twice now and it keeps slipping. My question is, Do I need to replace the hub? Or is could it be in the bearing assembly? Also, due to the cost I'm considering replacing them with manuel hubs. Any thoughts?