It sounds to me like the driveshaft is finished, it was probably already on it's way out when you changed the boot. If the old boot was leaking grease then there's a chance that the joint was running dry and thus wore out. Pick up a good pair of 2nd hand ones from a scrap yard and fit them both (i'm my own oppinion even if your other shaft is ok it's best to fit your replacement shafts as a pair because they will have equal wear.)
A pair of shafts should cost £20 - £25, but before you pay for them make sure you check them. Hold them from the middle in a vertical position and give them a shake, if either of them make a knocking sound then they're no good so don't buy them.
Hope this helps. (Y)