Read my post/thread started 4 years ago in this section-"4T65E Hard Shift-Solution"-all of it. Changing the fluid and filter will NOT solve a P1811 Hard shift issue alone. You must install a shift kit which replaces the shims and springs in the trans accumulator-accessed by dropping the trans fluid pan. I recommend a Transgo shift kit which is what I used. Also, you need to install a tranny cooler of some sort to reduce the fluid temps. I use 2 Long Double stack 18,000 GVW each coolers that cool the fluid outside of the radiator. Have not had a hard shift issue in 60,000 miles since the changes. You can comment on the thread 4T65E Hard shift-Solution or PM if you need more info.
