Problem bouncing up and down on highway?

Basically, my 92 does a very quick bounce on the highway once I hit about 75 on up. I was thinking maybe something to do with the wheels like wheel balance or something. But I did have new tires put on about a year or so ago and they were balanced and everything.
Thanks :)

"shaking" is usually u joints


...but your saying bouncing...im not too sure

If you have 3.73 rear axle gears and 26" tall rear tires, you'll find a bad driveshaft vibration at 75MPH roughly. Getting an LS1 Aluminum driveshaft is a good fix or an aftermarket piece will cure it. I went with a Spohn steel 3" unit. Works great! Goodluck!

New shocks would get rid of a bouncing motion also, if it's not driveshaft related.

Well yea, I was thinking shocks maybe too but I'm not sure they'd cause it.
Really, it is a really really quick up and down. I would have to say it is probably not a driveshaft vibration.

Now that I think about it, the shocks probably are on their way out. Well, when I get the money I might as well replace them anyways since it's been 130k miles. Thanks for the help guys. Keep chiming in if you have another idea.