I have a 94 D21 with the 3.0L V6 & auto-trans. 1st & 2nd gears seem to shift perfectly. 3rd & 4th gears however do not shift until around 4000 RPM and still sometimes require me to let off the throttle a little to shift. 3rd & 4th shift very smoothly, just at excessive RPM.
FYI: I just bought the truck. 180K+ miles and looks like the transmission had never been serviced. Fluid looked burnt (like tea almost) and it shifted very hard in all gears. Took it to a quick lube to have it flushed. Drove a little for the next two days and then dropped the pan, changed the filter & topped off with new fluid again. The high RPM shifting in 3rd & 4th are the only remaining problems. I have seen the check engine light two times, though just briefly. Brought it to my mechanic who plugged into the ECM under the passenger seat. No codes logged in the ECM.
I believe this to be either the TCM or the valve body.