I second Katavic918's description. Mine will shift as late as 28-30MPH but only if I have the go pedal mashed to the floor. I posted somewhere about it myself after getting my beast running and road-worthy and was told the same thing. In my case giving it an Italian Tune up seemed to help a little. That's where you push it hard, to a higher performance than normal driving. Not exceeding limits, just pushing harder than normal and making it work for a change.
I tried but couldn't find that thread. It isn't one I started.
If you have not changed your fluids and filter, I recommend you do that ASAP. Only then is the trans getting a more reasonable assessment.
Bulldogger