Automatic Transmission problems of the 1999 Chevrolet Express

After traveling for long periods of time on a highway and then slowing to a stop on the exit, the vehicle hard shifts in gears 1-2. If you shut off the engine, it appears ok until the next time. Transmission fluid is ok. Vehicle only has 47500 miles. Had mechanic check and gets computer code 1870, with engine check light on. Numerous posts on the web have similar complaints on chevy/gm vehicles with 4l60e transmission, but no recall to date. May now have to replace transmission.