After changing fluid and filters, solenoid pack, speed sensors, pressure sensor, accumulator plate and springs, the only thing that fixed mine was a transmission rebuild. They determined that there was a broken snap ring in the 2nd prime/4th OD clutch and planetary pack.
Mine acted the same way. It actually downshifted to 2nd gear once for a split second while doing 75mph down the freeway. It felt like I had yanked the emergency brake and them immediately released it. The slamming back and forth is the trans shifting into 4th, back to second, into 4th... If I turned off OD after starting the Jeep, it would shift the first 3 gears fine.
2002 WJ Overland - 4.7HO - 231tc - 6" Clayton - 285/75-16 Cooper STT - ProComp 5129 wheels - Kevin's Offroad Belly Skid and Rock Sliders - JKS Track Bar and Disconnects - ProtoFab bumpers