It could be an un-centred wheel, those desert rats are known to not centre correctly, my old ones used to do it if I wasn't careful.
Otherwise it could be a dying drive shaft, they go off balance as they age and can cause vibration.
Worn suspension bushes will aide a vibration.
You could have thrown a weight that's used to balance the tyre, you'll be surprised what can happen with that - it can come and go and there will be a certain speed that it will be really evident and speeds that you wouldn't know it was an issue... It could also be some mud on the inside of the rim.
Many things to look at!
Best of luck with it.
Cheers
Bennie