gear oil, reuse or not

Probably, you'll know when you try and bolt the retainer to the flange, if it doesn't contact the flange by hand, then the axle probably isn't seated. DO NOT try and pull it in with the bolts.

Are you running the stock Timken tapered roller bearings?

Also, check and make sure the thrust buttons or block is in the suregrip, if it is, and you have OEM bearings then you're good to go.