Transmission problem or not?

I have a 2002 hyundai XG350 with n80,000 miles. I have no problems shifting from park or drive but when the car goes more than 50 miles a hr on the highway it sounds like its straining to go into the next gear but it doesnt go into the next gear but the RPM stay the same about 2000 rpm I checked the transmission fluid level and it looks okay what could be causing the problem