Don't have to drop the t-case to change out the output shaft seal.
Do you still have the bell around the tail shaft housing? If so, take out your driveshaft again, then knock that bell off (it just presses on, hit it from the back side with a hammer or other implement) and then recheck that the seal is in all the way. If it is, then replace the seal again. But when you take out the old seal, check that the area where the seal sits in isn't nicked. If it is, then spread a bit of RTV around the area (I go all the around with a little bit of Right Stuff for insurance) and then knock in the new seal. Leave off the bell, you'll end up denting it at some point and it will make noise, so just lose it anyway. With the new seal in, put the driveshaft back in and drive it.