98 stx, 4.2, auto. Just had the transmission rebuilt at 60k miles. Between 63-68 mph, theres a low hum/vibration. This is when accelerating through those mph, or sitting with cruise control on, at any of those mph. This is in 3rd or od.

The trans was so sloppy before the rebuild that it's hard to say if the vibration was there before the rebuild. I did a tuneup and had the tires balanced. It's still there. As soon as I hit like 69-70 it gets smooth again and the hum goes away.

I did a search, but didn't find anything similar. Thoughts?

 

It might be the tires out of round? Just a guess.

 

Get under your truck and look where the driveshaft bolts to the rear end.Is there a mark on the driveshaft in relation to the differential yoke? In other words,before removing the driveshaft they should have marked it in relation to the yoke. If there is and the marks are not lined up then unbolt it and line them up.If not,then index your driveshaft;unbolt it and turn it 180* and bolt it back up.If that dont do it then have your driveshaft balanced.