Rear end shaking and sounds like a grinding noise inside the car

I bought my ford explorer back in 2007. I haven't had to do any work to it except for the normal stuff. Like oil changes, tune ups, breaks, tires, etc. After about a year or so, the rear end started to shake real bad. You could feel it inside the car, but it would only do it in the winter time as I recall. Cant quite remember but something like that. Now almost 7 years later it has gotten worse. Everybody that gets in my car is like." Are we going to make it?" Or "Is the rear end going to fall out?" Its really that bad. So I took it to a place called Discount Tires, and the pretty much guessed and said that it was my wheel barren on the right front. So I had them replace it. Then I left still doing it, so I returned the next day. And well they said it was the rear driver side wheel barren so I had it replaced. Drove off it still was doing it. At this point I am so mad and have no clue why they couldn't tell me what was wrong and why they were just wanting to replace whatever they could. Made no sense to me. Anyways. I took it finally, to a place I have gone to in the past and they said I had the wrong type of oil in my rear end differential (CHUNK), and there was too much oil in it. So they told me to replace the oil and add an additive. So I did all of this. The noise and shaking stopped for two days now its back. Please help me figure out what this could be. Do I need to replace the pinions and stuff inside the rear differential (CHUNK)? And if so will this help? Also, one more thing, Is this hard to do yourself, or Is this something that needs to be taking to some mechanic shop to do? Please let me know what I can do.I have spent a lot of money and running low on funds for this.


thank you so much, angela