2004 Sierra crew cab 1500 Z71 5.3

I replaced the encoder motor sensor about 5 months ago (5000 miles) and when I opened the encoder motor it was full of oil from the transfer case.
I replaced the sensor anyways and it lasted until this morning when the lights on th dashboard switch say the transfer case is in neutral. and I get the "service 4wd" again.

I read somewhere that you have to spit the case to replace a seal in the transfer case shaft goiniig to the encoder motor.... is this true? I cant really see up there too well but if i can just drop the case and pull the seal... Id love to do this. I have a long trip (1200miles) and ill be towing my 5000lb trailer on it in two weeks.

Your thoughts?

Morgan Trotter