Bad Vibe

The rain has finally cleared here in so cal. and it is Friday I
just took out the duster, its been sittin' for about 2 monthes.
It fired up and I'm driving around to some of my buildings,
I am noticing a bad vibration on deceleration in all 4 gears.
I replaced the driveshaft and rebuilt the 4 speed a few
monthes ago, w/ new u-joints and now I am getting this
vibration. I have been on quite a few cruises up thru the
mountains and all no problem. This just started today, I know
the rear-end has never been serviced on the car and I have added
gear oil to it. What I drained out of the diff. was pretty bad.
Does this sound like a differential problem? I've never
noticed this before & its a pretty bad vibration.
Thanks for the help guys!! 8)