Front differential leaking fluid where driveshaft goes in. Sportsman 500 HO 2013

I just replaced the front differential on my 2013 sportsman with a used differential. After a 5 minute with the new diff I noticed it was slowly dripping from where the driveshaft goes into it. I’m assuming I’ll need to replace that seal. Will I need to take the diff right back out to replace it? And is it OK to drive it as is if I keep it in 2wd?