It may be a clutch issue. It appears this model has the E-Z Link type split boom from the diagrams I have seen. Most trimmers except cheap ones have a clutch. When you reassembled did you remember to tighten up the T-20 Torx screw inside the clutch drum to the end of the crankshaft? Did you remove and re-install the clutch itself on the crankshaft? Did you properly install the large washer that goes on the shaft with the clutch?
In lieu of a broken upper drive shaft, check it and also check the drive shaft of your attachment(s) to verify they turn freely. If the upper shaft turns freely by hand then the clutch may not operating correctly when the unit is running. post back and let us know.