1976 K20 sub 205 transfer case shifting

[orange72sub]

Hey guys need a little help. I purchased a 72 K20 sub with a 76 K20 sub drivetrain and frame under it. The truck runs good and shifts fine too, but when I go to put it into 4hi it won't allow me to do so, I can how ever shift it to neutral and 4low and back to 2wh but not too 4hi. I am assuming that the shift pattern is all in align and 4hi is straight back to the seat. The truck sat for a long time with out being shifted into gear even before I got it. The transfer case also has a bad case of the drips do I have to pull it to replace seals? I have not worked on a transfer case before but I am pretty capable of taking on a task with the right manuals. Any help would be great thanks!