Has it been like that since you got it?
I never heard of a flywheel become "unbalanced" but it could be slightly bent.
Just a thought.
Or maybe the mechanic that installed it was a little unbalanced.
Another thought.......I don't know the history of your truck but I ran into this in an S10 Blazer. Some of you may have read one of my rantings on this. The so called mechanics that installed the rebuilt motor in this Blazer only installed three bolts in the bellhousing and even two of those were loose with one ready to fall right out. The truck vibrated a little when he 1st got it back but got worse and worse until it was vibrating like a futher mucker. We thought it was in the tranny, like some flywheel bolts loose or something, so we began preparations to drop the tranny. About ten minutes in I noticed the 1st missing bolt which is like a beer, you never have just one.
Once we replaced them it was fine.