HELP! Serious ViBrAtIoN after service!!!

[TroysH2]

After 5 visits to the dealership for a driverside front differential seal leak, they agreed to fix it. The mechanic said that the whole assemble would have to be ordered and replaced (not sure what he meant by "whole assemble"). After waiting 1 1/2 weeks for the part/s, they made the repair.
Now the new problem. On my way home the H2 had a serious vibration at all speeds. It feels like I am towing a heavy trailer, and has problems going up a hill. The engine sounds like it under strain.
Took it back to the dealership and the mechanic drove it and said that there was a "harmonic" problem with the shaft. He also told me that they only replaced the seal and not the entire assemble.
They keep it for two days and the service manager told me there is nothing wrong and that the vibration is normally for GM products. He then told me he would fix the problem if somebody pays for it. He said my next step was to talk with the GM rep. when comes to town(5 weeks from now).

My Question is; what might be causing this vibration, and what are the possible solution/s.
Note: I have been driving my H2 80-100 miles a day for almost 2 years. Currently over 37,000 miles and was under 36k and warranty when this all started.

Thanks for the advice in advance.
troy