Car Shaking on acceleration between 70 & 100Kms/hr

I just bought a 2007 suzuki swift yesterday, private sale, and didn't test drive it at highway speed. Took it on the Highway today and discovered when you get up to around 70kms an hour if you are accelerating the car shakes quite violently, take your foot off the accelerator and it is not as bad and once you get up to 110 it is not very noticeable at all. Also tried going down a steep hill at 70 with my foot off the accelerator and no shuddering. Any ideas? and if so cost of repairs. It has only done 28070kms.