Is it a droning noise?
Did you jemmy the diff about to fit the new bushes?
If It wasn't making the noise before you changed the bushes, then it has to be something you did whilst changing them.
My dad did mine thinking he was doing me a favour. He pry barred the diff over to make getting the bush in easier and I ended up with a annoying rotational knock at about 30mph +, this eventually turned into a drone starting at the same speed and I have just had to change the rear diff driveshaft carrier bearings which has cured it. if one of your drive shaft oil seals starts leaking then there could be a good chance it may be this.
My first port of call though would be to drop the entire prop shaft to rule out vcu bearings and if it's still there I'd look more closely at the diff......if it is the carrier bearings it would also pay to chage both rear wheel bearinfgs at the same time as 90% of the stuff to do these is removed for doing the carrier bearings