Shrimp300: what year is your car? If it is 91-93, then the computer controls the 3-4 upshift. The transmission on this model has 3 wires going to it, 1 ground, 1 for the torque converter lock up, and another for the 3-4 shift. You may have a break or unplugged wire/connector.
The transmission otherwise shifts correctly? If so, I doubt it is a vacume line or modulator problem. An poor vacume source would cause a delayed and hard 1-2 shift, as well as for other shifts.
As others said, check the VSS sensor and wires, along with pulling codes on the computer.
If your car is an 87-90 and doesn't go into OD, then it could be a sign of more serious problems, but also true for the 91-93. It could be a shift linkage out of adjustment, or just a sign the transmission has problems and is giving up the ghost. This transmission will often lose OD first as it goes out.