slow to change gears going uphill

I have a 92 sonoma with TBI. It runs great except when you start up a hill. If you start up a hill running at 55 mph and keep the throttle at the same position it will slow down to about 35 or 40 before it kicks into a lower gear. Other than that it runs great. Only has 70,000 miles on it. Gets good gas mileage. Any suggestions? Thanks, John

Try checking the level of your trans fluid. If the level is ok, you might want to have it changed.