Van jerks really hard, it made a loud whining sound transmission does not sound good

Unless there is something I am missing, putting the shift lever in "third" only means that is the highest gear it is going to go to. That is recommended for towing a trailer. In "third" or "O", it always starts out in first, then up-shifts at the appropriate times. Driving with the shift lever in third gear will only avoid the problem from showing up if it is related to slippage in one of the clutch packs.

The concern is the defaulting to "limp mode", (second gear). That is to let you drive slowly to a repair shop without needing a tow truck. The Transmission Computer does that when it detects a problem. That problem could be electrical in nature or mechanical, meaning something inside the transmission. To get it out of limp mode you must turn the ignition switch off, then restart the engine. Your mechanic read the stored diagnostic fault code(s) to know where to start the diagnosis, and it sounds like the problem has been solved.

For future reference, sensor circuits and other electrical circuits are monitored all the time, and the system will go to limp mode as soon as a problem is detected. That means you could start out in second gear as soon as you shift to "drive" and start to move. When a mechanical problem is detected, like slipping clutch packs or a pressure that did not change when a shift solenoid was energized, that will send it to limp mode when the problem occurs, and that means after you are moving. Most commonly that is during or right after an up-shift.

As for your worry about needing a new, (rebuilt) transmission, your mechanic or your transmission specialist can use his scanner to read the "clutch volume index", (CVI). That is a set of four numbers showing the volume in CCs of fluid that it takes to apply each of the four clutch packs. As the plates wear down, it takes more and more fluid to fill in behind them before they lock up. Based on those numbers, an experienced specialist can tell you the percentage of life left in each clutch pack. The computer updates its shift schedules every time a little wear takes place so you always have a nice crisp shift like when it was new. The disadvantage is you don't feel the sloppiness occurring like we did a few decades ago that gave you a year or two of warning that a rebuild was in your near future. That is where the CVI numbers can tell you if there is a failure related to the clutch packs coming soon. There are plenty of other things that can surprise you though that cannot be seen on the scanner.

There is one more thing I forgot to mention and it is affecting my 1994 Grand Caravan right now with the same transmission. Once warmed up, it shudders and jerks really hard between thirty five and forty mph but only under very light throttle. That is the speed at which the torque converter locks up for better fuel mileage, (another Chrysler innovation (1977) along with the computer-controlled transmission (1989)). The chattering is caused by the use of the wrong transmission fluid. Normally that does not send it into limp mode, but it sure is not comfortable. The previous owner said this problem occurred every year and was solved by changing the transmission fluid and filter, but they were not aware the specific Chrysler fluid had to be used. It is not the fluid that is different. It is the additives in it that are different. The wrong fluid has too little of the needed additives and it wears out very quickly.