Truck Repair: Noise and vibration from rear, happing, snap ring

Question
I have a '99 GMC ext. cab swb 2wd truck and I have about 89,000 miles on it. I have noticed I have a noise and vibration coming from my rearend when I start off from drive, but as I speed up to around 45 or 55 mph it stops but as I start to slow down to around 35 and less it starts alot of vibration.
I just had one of my U-joints replaced over a year and 2 months and I just made a trip about 1300 miles and this all started happing after I got to our vacation spot for a week the first 500 miles. I made the trip and home again but didn't think I was going to make it home without a break down on the way back. What could be wrong?  

Answer
Whoever replaced the u joint may have not gotten a snap ring in just right, and the joint may be shifted sideways.
If you can jack the rear off the ground and block it up, run it about those speeds and see if you can narrow it down, but be careful near moving parts.

Van