Your subframe mounts are not causing the driveshaft problem, because the engine sits on the subframe, not the shell. But I do reccomend that you change them asap, as they must really be ruining the handling. If you are not in a position to fit a solid kit, use standard rubbers for now. Poly are useless here, and as you may know, you must never mix them or bad things, like shell cracking, will occur.
I do recommend that you try a new pair of engine mounts to see if they solve your driveshaft problem. They should not be terribly expensive, but you will need some patience to fit them, as they are a bit awkward. I prefer to jack the engine and take them both off, fit the clutch end mount first, and finish with the radiator end, as there is more room at that end to persuade the last two bolts into their holes. They never line up properly, because the new mounts try to make the engine sit too high at first, and are too wide to fit between the sloping subframe faces without persuasion. Having the grille and radiator out helps immensely, but it can be done without.
I always weld steel self-locking nuts (the Philidas nuts used on early drive shaft U bolts are ideal) to the mounts prior to fitting. If you are quick, you don't damage the rubber, and it means that the bolts go in from underneath, which is easier. Using Philidas nuts, the bolts will not work loose and fall out. I just don't have the patience to fit those bolts from above, unless the entire engine and subframe is out of the car (as it was in the factory when they were fitted that way).
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