3rd and 4th gears not working properly

If I get it up to highway speed in second and then shift to third and take my foot off the gas, the revs will come down to what they would be in third gear at that speed. If I shift into overdrive they drop again accordingly. There is engine resistance (more in third obviously), it's not as though it just goes into nuetral. But if I give it gas in either third or fourth, it just slips. I have to drop it to second to get it to pull at all. Any ideas about what I'm dealing with here?






