Hello, im not trying to get an exact answer just some possible culprits. My dad was driving his 1988 k2500 with the 700r4. He said it felt like it was slipping on the 1-2 shift. A few miles later it lost all gears and rolled backwards when you gave it gas. The transmission was a re manufactured unit from gm that was installed maybe 4 years ago. what are some common issues or parts that could cause this problem?