You might raise it up, blocked with some good jack stands, chock the front wheels and let it idle in gear. Search for your vibration with a mechanics stethoscope. (it would be at a much lower frequency at idle speed)
Low on lube, bad axle bearing or carrier bearings. Let's hope it's something like a U-joint as that's gonna be cheaper.....
C-clip diffs are hard on axle bearings because the axle is the inner race.
Let's hope for