F150 1996 Manual trans

Vibration on and off, more at hwy speeds
And when coasting or going very slow less than 5mph I hear a thumping sound, I swear its in the rear end I thought it was a tire .
I have changed u joints they were good any way .
I have replaced a bent wheel in the rear .
I have changed tires and balanced them.
That made some difference on the vibration but I still have this thumping sound .
You can only hear it on smooth surface.
I really can not tell you when it started because of the snow has not been gone that long in Michigan .
Any ideas out there ?
Im thinking about changing the fluid .
One guy said it might be the carrier bearing, but he did not ride in my truck .
He just went by what I told him was happening.