automatic transmission won't shift from 3rd to 4th. i have to shift it by hand. i replaced trans vacum modulator same problem. also when on freeway it shifts back down to 3rd sometimes. when i remove vacuum hose from modulator it has no problems shifting thru all gears but it shifts hard, what can be the problem? can i run my car without vacuum modulator? any input would be appreciated.

Did you ever figure out why it was doing this, or what the problem was and the solution?

Thanks!