I've heard of guys putting a small amount of Berrymans Chemtool carb cleaner in the oil and running it for a few minutes at idle to clean the oil system out. I think there are instructions on the can telling how to do this.

I'd say based on what you said above, before tearing into the motor its a good idea to go ahead and do a good flush on the motor first. I'm all for taking the simplest route and who knows, it very well may turn out to be crusty oil in there causing the problem so there's one way to find out. Be sure to flush and fill the differential while your at it.