The flex discs cannot be oriented incorrectly, so that is not likely your problem. Driveshaft balance is an issue on these cars, and can be very difficult to correct once it gets out.How should the rubber guibos be in relation to the driveshaft? I think it may be coming from the middle guibo... !
There are a couple of things that can happen:
1) The driveshaft halves can be re-assembled in a different orientation than they originally were. Usually one would mark them so this doesn't happen. Also, there are specific numbers of washers and nuts on each bolt on the driveshaft - these may or may not contribute to the overall balance, but it is a good idea to replace each set of washers/nuts in EXACTLY the same spot it came from.
2) If the car was driven for a long period of time after a flex disc "blew up", the yolks and splines can be damaged.
A good next move would be to remove the driveshaft and take it to be balanced. Not every driveline shop can balance a driveshaft that spins at engine speed, so ask around in your area and find a good one.