Welcome to the forum Pup. Your cars not a pup, just needs a lil' bit of TLC.
If it was your drivetrain/driveshaft (ie the very long cf bit, not the axle assemblies you lot refer to drivetrain ), it would most likely be the Universal Joint (UJ) you would feel the vibration very heavily through the firewall under load if you put your ankle against the firewall, you would also feel the vibration through the seat and steering wheel. Axle assemblies can vibrate, but given the first description you gave us in post one, check the things RTbiscuit has suggested, seems most likely. (I've been through replacing the driveshaft & an axle assembly quite a while back).
Mr Biscuit is onto it. Let us know how you get on.