09 Subaru Impreza wagon centre diff ratio

There's no way to "check" the transfer case. What happens is that the Teflon coatings on the bearing seats are worn off. Try adding a small bottle/tube of TransX Posi-Lock (or similar name) gear additive after changing the old ATF from the tranny. Within a couple of figure eights you might notice the "crabbing" go away. If it doesn't within a day or two you may have ruined the transfer case. Some cheap wrenches use a shot glass of brake fluid instead, but better to use the Posi-D additive. Works wonders...but do start with clean ATF, as even dirty ATF can cause the problem. If yours is a STICK then the problem is invariably a worn-out center diff called a "viscous link", that freezes up instead of loosening when turning. It's a single $500 or so "pumpkin" that's pretty easy to swap in behind the manual trans. Good luck. Ern