So my gear stick shakes like mad above 30 mph.

Is this normal or do I have a worn mounting somewhere.

Any advice greatly received.

Is gear changing or gear stick position also a bit vague? If so I'd look at the bias springs under the gear lever top. Easy to fix if the springs or the bias plate is broken, search the forum for many write-ups.

I'd also, while I'm looking, check the state of the nylon ball at the base of the lever.

If all is normal, I'd check the gearbox mount bushes on either side. Heavy job ...