Question about shifting into highest gear

My Truck is a 1995 Tacoma with 230K. It runs great.
In any kind of weather my automatic transmission will not shift itself into high gear until the truck has been running for 10 minutes or so. After that no problem. This has been happening since I've been driving the truck-almost two years... I'm selling it soon and want to be able to tell folks about the "problem". Any Ideas as to what is happening?

 

I've never had an automatic Tacoma so I'm not sure if it had the same features of the newer auto trannys. I know on the wife's 07 Yaris it doesn't shift through the full range until warmed up. Kind of a pain but it does get the engine at operating temp faster.

 

when was the last time you had the atf flushed?

cooler in decent order? maybe syncros getting a little worn?

 

Do a transmission flush. Mine only does it in cold weather, which is normal. It could also be speed related, they don't shift until they get past a certain speed.