'92 Toyota O/D shifting problem

I have a '92 Toyota Pickup. For several years now it has been taking a
very long time before it will shift into overdrive. Based on what I've
read on the internet, Toyota uses this feature to control emissions,
however, my truck takes an inordinate amount of time to reach the point
where it will shift into OD... I can let it idle for 30 min, and it
will still take 3 miles or more of running at 4000 rpm before it will
shift up. I took the truck in to the toyota dealer, and they claimed
that there was no problem; I replaced the thermostat, and it still
hasn't helped. What else can I try to get this fixed?

Thanks for your help!

I'm experiencing similar problems, and right now it's leading me to
vacuum hoses. My post is under yours a few, and that's what everyone
seems to be pointing their fingers at. I heard there might be a
solenoid that controls the overdrive, haven't had much time to check
into it yet.