Diff Cranker

l got an 83 4x4 with 250,000 miles on the trany. lt still shifts smoothly and stays in gear but getting into gear from a dead stop is a problem. lt will not go into 1st gear unless l hold the clutch in for about four seconds or if l tap it into forth briefly. l'm guessing the syncros ring for first is out and can't grab the spinning shaft. lt's the same for 2nd, 3rd and 5th. l'm considering to rebuild the whole trany this summer but would appreciate any advise or answers on what's causing this problem.