1991 Mitsubishi 3000gt feels like slack in drive shaft????

[cowebb1]

I bought a 1991 Mitsubishi 3000gt SL for my son for Christmas. When taking off it is perfect. It seems tight and has no knocking sound. After getting up to speed there is a hard bumping, seems like in the rear, and it feels like slack in the drive shaft. Does that make sense? I can not find a Chilton or Haynes book on it. Does anyone know if it is the drive shaft and where I can get instruction to replace it if that is the problem? It has 109,000 miles on it. manual transmission. If gearing up or down where there is pressure no the drivetrain, there is on issue. Thank you so much for any help you might give.