Transmission issues

Car will not pull when put into drive or reverse. When put into gear you can see the tach rpm's drop and feel the car "try" to move, but will go nowhere. Have changed transmission filter and there was no "burnt" smell to the fluid, and what appeared to be the normal amount of material on the magnet in the pan. Have also changed the neutral safety switch (just in case. Don't know if it has anything to do with it, but it was cheap) but no joy there either. The last two things I can think of (before a "new" transmission) are either the modulator valve, or a horrendous air leak in the vacuum line going to it.