GM-GMC: vibrating truck, wheel seal, axle shaft

Question
i have a 97 chevy duelly, i had the axel seal replaced and ever since that when i get up around 55 mph my rearend starts to vibrate really bad,i took it back where i had the work done and he said it was a bent rimm so i put new tires on it and new rimms thinking it was one of those but it is still doing it, what else could it be?

Answer
Which seal?  There are three:  One at each wheel and one where the driveshaft attaches to the rear axle.

Changing a wheel seal should not cause a vibration like you are describing.  A dually has a full-floating rear axle, which means that there are two bearings at each rear wheel (just like the front wheels on older cars).  The axle shaft does not carry any weight.  It only turns the wheel.

If the driveshaft seal was replaced, the driveshaft may have been installed 180 degrees from where it was originally.  This could cause a vibration that feels like it is coming from the rear axle.

The shop that did the work is responsible for the vibration and they should correct it.