I'd definitely do what Trendz suggested! I forgot to mention a couple years ago I also had a vibration that turned out to be twisted axles. Again, wasn't noticeable at speed. I found it by letting the car idle in gear on jackstands.
https://www.youtube.com/watch?v=u7en1p9e7iU

CHIP
'70 hemicuda, 575" Hemi, 727, Dana 60
'69 road runner, 440-6, 18 spline 4 speed, Dana 60
'71 Demon, 340, low gear 904, 8.75
'73 Chrysler New Yorker, 440, 727, 8.75
'90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt
'06 GMC 2500HD LBZ Duramax