Start pulling things apart and feeling the bearings to see if they are gritty, hang anywhere, sloppy, or if any sound different.
What I do is pull the motor out and center diff. The center diff bearings are easy to check and you've then isolated the front drive train from the rear.
Start spinning things and see if it's free and listen for noises. A bad bearing is typically pretty easy to spot.
For replacements, you can't beat Avid RC. Great bearings and super cheap.