Rumble (vibration) that scares me

I have a 94 gt and it is an auto. (i know should be a standard) but lets get past that.

Anyway driving today and I am going about 50km and I notice a rumble (vibration) that is coming from my backend. At first I thought I rolled over rumble strips but a little later it was there again. I gave it a little gas and it fixed it self temp. But now it is happening more often. I just ran to the store and it did it again. Is it my gears or could it be the driveshaft out of balance?
Only seems to happen when it is just about to change gears, when it wants to shift hence give it a little gas and it corrects itself. Any thoughts......I don't want to drive it now becasue I am afraid of doing more damage possibly, which sucks cause it is warm here and girls are out and in practicaly nothing....God I love summer.
 
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