I bought a JDM GSR Tranny which has no label on it, the only stamp is "YSK-1" on it
and it grinds in 3rd after 4000rpm
I am going to rebuild the internal with brand new seals, bearing, and synchros..

The thing is: How will I know the differential bearing whether it is ball bearing or tappered bearing?

A direct method is to open up the case and see, but it is currently on my daily driver so the worst thing is to order all the parts and rebuild over the weekend.

Any idea what I can do?

Thanks,

 

Depending on the year, it could be either. Pre-96 they are all tapered bearings. So if you don't know the year of the trans, you're kinda beat until you open it up.

Chances are though that the diff bearings are just fine, and you'll need a special tool to measure the preload on the tapered bearings, so you may want to just skip on the diff bearings unless there is a serious issue with them.