uncool vibrations

[milesstarkey]

i moved my axle back but then the new fuel cell wouldnt fit so i moved the axle back to where it was. i had the driveshaft professionally resized and now i have a vibration between 15 - 40 mph (approximately since i don't have a speedo). any ideas as to what i should check could be the problem? the shop says it can't be the driveshaft cuz they 'don't make mistakes'.

 

[CruisinGA]

You installed it out of phase, or the shop put it back together out of phase. (I doubt it).
Do you know what "in phase"/"out of phase" means, or do I need to explain it?

http://www.4xshaft.com has a good explanation I believe.