Trans has problem shiffting

I have a 1991 chevy s10, 4.3L and auto trans(700R4). The engine has around 50,000 miles on it because it was completely rebuilt. The odometer says 202,000 miles. Im not sure how many miles are on the trans. It shifts into all gears smooth but when shifting from 4th to overdrive, it takes a long time. It does'nt shift into OD till around 55 - 60 MPH. Sometimes if I left off the gas a little bit it will shift but not all the time. I changed the fluid and filter about 15,000 mi. Ago. A few years ago it would'nt shift out of first gear, until I went down a big hill then it shifted fine. That is when I changed the fluid and filter. I have not done any modifications to it.
Thanks for the help