Yes the diff will be very warm to touch after a run, sometimes in the rain you can see steam. Don't assume though that the oil level is OK just because there are no leaks. I presume being a bus it is a twin wheel version. The brake drums are sometimes very hard to remove because they seize onto the hub. To get them off you have to undo the wheel bearings and remove the halfshaft and then the hub/drum in one assembly. Doing this often causes a loss of some of the oil from the axle tube. It would be best to check the level on flat ground in case this has happened before you bought it.