I have a 2000 Nissan Xterra slipping between 3rd and overdrive

Hello, you could very well have an internal mechanical concern with the transmission. A diagnostic tool will help see what gears are being called for and actual gears engaged when the problem is occurring. Changing parts is a bad idea without seeing what is going on. Invest in a good diagnostic test with a qualified local technician. https://repairpal.com/repair-shops/nissan-repair-and-service-in-newark-ohio