The 3-4 shift, as well as the torque converter lockup, is electronically controlled by the PCM. There's a fair chance that the problem is external to the transmission in the EEC-IV system. Before turning my attention to potential failures internal to the transmission, I'd make sure the electronics external to the transmission are working.
Start with the EEC-IV "quick test" (see
www.dalidesign.com/hbook/eectest.html for how-to. Consider going to the library to see if they carry
Mitchell, where you can find a more complete EEC-IV diagnostic article with circuit tests and such for diagnosing any trouble codes that come up) Fix any trouble codes that show up, since the PCM uses information from the engine sensors to decide when to shift from 3 to 4.
Then perform the output state test (PCM goes into output state test after it output KOEO and continuous memory codes). Get a wiring diagram, so you can identify the 3-4 control circuit wiring and see how it's put together, and verify that the PCM can switch the 3-4 shift solenoid on and off.
If it's passed the tests up to this point, perform a driving test. While monitoring with a DVOM the 3-4 shift circuit, drive the truck and see if the PCM is telling the transmission to shift. If the PCM is telling the transmission to shift, and the transmission isn't shifting, then the problem lies inside the transmission (valve body, od clutch or what have you).
And, one last note, be sure to distinguish between 3-4 shift (overdrive shift) and torque convertor lockup. I see some people who confuse overdrive and torque convertor lockup as being the same thing, but they aren't. The 3-4 shift circuit and the TC lockup circuit are almost identical, so diagnosis is the same for both. I jsut mention that to make sure that you are looking in the right place.
