Driveline questions

I just posted this but had the wrong title for it
Ok guys. Recently I have noticed a suttle vibration that almost feels like the same vibration you get when you are Idling too low. However I feel it at about 7 miles an hour wether it's in neutral or in gear with the clutch pushed in. I also noticed a squeaking coming from my drive line right at my rear differential that goes away when I get to about 30 mph. I discovered one of the four seals in my rear u joint has started coming apart. I'm assuming the squeaking is coming from the u joint. When I grabbed my drive line and rotated it, there is a decent amount of play in the rear end. My questions; is it normal for a u joint to fail at 65k miles on a 2007 truck?? Is it normal to have play in the rear differential where it attaches to the drive shaft?? Any thoughts