Transmission taking long to change up.

[BLUETHUNDER]

Ive noticed for a while now that from 2nd gear to 3rd seems to hold on far too long before it changes. Anyway on the freeway yesterday whilst driving along "Transmission hot" came up on the dic. A quick glance at the gauge and the temp was heading past 200. I pulled in at the nearest service stop and could tell it was running hot. I could smell the type of smell when someone slips the clutch on a manual car. I let it cool for a while. And the attempted the freeway again. It wasn't long before i figured what the problem was. With the Truck at 60mph it was spinning at 2200rpm and the temp rising again. It was stuck in 3rd. Letting my foot off and on the gas pedal or shifting from D to N and back to D again shifted it into top and resulted in the temp coming down. Does this sound like a blocked tranny filter. Or could i be looking at worse. Just to let you guys know. I dont tow with it. And i doubt if any of the previous owners have towed trailers with it.