Vibration - Suzuki - Xl7 :: 2002 - Shaking As Go Past 55 Mi/h

I've had my 2002 Suzuki XL7 for 7 years and about 3 months ago, I noticed that as I go past 55 mph, my car creates this vibration that almost makes me think that it is trying hard to run.

Took it to the mechanic and they diagnosed after test driving and lifting it up that the wheels needed replacement. As it turns out when the car was running while lifted, the rear driver's wheel wobbled. The tires were fairly new, about 3 years old. They suspected some cord separation inside. I went and had all four wheels replaced but the vibration was still there. I took it back and they think rear differential is the problem, either the bearings need to be replaced or the whole differential axle.

The mechanic drained the differential fluid and showed me how it was almost metallic in color. The cost of replacing the bearings is $1100; to replace the whole differential with a used 88K miles, no warranty is $1400 and $1700 with warranty. What do you think is another possible diagnosis for it. I just did not want this to be another wheel incident where it didn't solve the problem. I heard that since it's a 4WD that it may be the drive train also.