When that happened to mine, it was because my rear driveshaft was too short. It stressed the main shaft in the transfer case too much and bent that, also cracked the tailshaft housing. I ended up replacing the transfer case and lengthening the driveshaft so it would grab more spline.
About a year later I had a leak between the tranny & transfer case, too, but the fluid didn't travel so far back. With that and the driveline vibes I've had, I decided to go with the JB Conversions SYE kit. I just got the transfer case back in 2 days ago, and ordered a Tom Woods custom shaft yesterday. While I was doing this job, I noticed some things that could have contributed to the leak issue. The brace from the transfer case to crossmember had a broken bolt where it mounts to the bellhousing. 1 of the 4 bolts that mounts the case to the adapter was broken off, the other 3 were pretty loose, and the tranny mount was shot. Upon reassembly I used plenty of Loctite. Replacing the seal between the tranny & case was not a big deal - hardest part is getting the transfer case back in IMO (I had to have my husband help with that).