904 won't go into passing gear...

I am having problems with my 904 tranny in christy(95% Christine + 5% Dodge = 71 swinger). It seems to shift fine around town but on the free way it doesn't seem to want to go into passing, occasional it will slip. The transmission was replace before she went into the garage for 7yrs. I believe it is a 904 seeing that the car is a high option 318 with a 8 3/4 rear. I haven't screwed with the linkage yet seeing that there are a million steps to doing it. I am not sure what it could be and if anyone has any ideas it pls let me know. I don't believe it has had a tune up since it was replace and I'm not sure if it needs one after its been rebuilt.