There's actually quite a fair ammt of both. Lot's of CRP land too. My uncle lives on the Tucannon past Last Resort and tresspassing is getting to be a problem down there, so if you have an invite I'd take advantage of it- esp if it's up high on the plateaus where the Mulies are.
I've hunted modern there for whitetails but stopped after some a-hole almost shot my buddy- we were walking out of an alfalfa feild to the truck when this guy started taking shots at a running doe accross the feild- my buddy was between the doe and this chump.. funny thing is that this guy started yelling at US. So that was enough of that. Lot's of guys just road hunt that valley and shoot from the road, accross feilds, next to houses, etc. I guess my point is to steer clear of the valley/whitetail areas- it can be a zoo.
Get permission from a wheat farmer to hunt for mulies and you will do well.