I have been searching and did not find a similar issue.
My 1998 with 2.5 and auto just started to jerk and buck at about 50 to 60 mph. About the time it goes into OD. It will do it nearly every time. Sometimes it does it several times and then others it will do it 2 or 3 times as it it speeding up. Above 60 and below 50 it is perfectly normal. No loss of power, no CEL, and still gets 20-22 mpg.The bucking and jerking is very quick.
This started about 1 week ago. Yesterday I did the pan drop with filter change and 4 qts oil. The oil was not bad and not burnt. (I have seen much worse!) I cleaned the pan and magnet. Magnet had normal amount of junk on it, no big pieces of metal at all. Just the very fine particles. The strangest thing to me is it has never done this in the just over 1 year I have owned it and then from one day to the next it happened.

 

After doing more research on the net it seems like this happens a lot and no one has a fix. Most people jump to engine misfire. I'm 99% sure I do not have a misfire. RPM's stay very consistent. I am actually leaning to U-Joints. Yesterday and now this morning I can hear a tin type rattling sound when I take off from a stop and it increases with speed then goes away. I know the dynamics of the spinning driveshaft change with RPM and load. I also noticed that it does not have to be in overdrive to have it happen. Just in the speed range and under at least a small load and it immediately goes away when you let off the gas (Dynamic change in DS)
So I'm going to try the U-Joints tomorrow. With 221k miles they are probably original and need replaced anyway and are an inexpensive guess!

 

Nope that didn't work!! LOL
Maybe it is an engine miss under load at those speeds.