203 transfer case problems

[Dmax21]

I have 73 4x4 with a 203 transfer case with lock out hubs and I think it may have been converted but don't know for sure, anyways I was using 4x4 and while driving slowly the front wheels locked up and would not free up and I couldn't get the transfer case back to 2wd so I kept trying to backup and go forward and it freed up and made a pop sound and now the 4x4 will not work, I can lock it into 4lo and it will reduce the gears but no power to the front wheels. Now I haven't tore into anything yet but what do you guys think the problems could be or could have caused this? any tips would be great.