97 f150 4x4 rear wheels won't pull
 

For awhile, when put in drive had to rev engine a little to get it to go into gear. Now, won't go into gear in 2x4. In 4x4 front wheels pull. I figure it is something in the transfer case, but don't know what. Any ideas? Or do I just need to pull the transfer case and take it apart to see what is broken? Rear drive shaft doesn't turn in 2x4 so it isn't the differential.
Thanks