bwstout

I bought a Colorado 4x4 in January 2005 I now have 32000 miles and it developed a vibration from 20-30 and 50-60. It is more noticeable in 4x4 than 2 wheel drive, but you can fill it in 2 wheel drive.
I have taken it to the dealer 3 times they put anew drive shaft it but it did not help it. They told me that I was driving it in 4x4 too much. Dealer said that I should not drive it faster than 20 mph for about 5 miles at the most. Question? What is 4x4 used for? I just need any help that someone might have.