shutter/vibration

I just bought a 02 Ram 1500 with the 5.9L automatic. When traveling down the freeway at about 65-75mph, in overdrive, with engine load, it shakers and shutters really bad. The guy told me when I bought it that they replaced the engine heads due to a misfire. It seems to run fine at all other speeds. The tranny seems to sift fine. Any Ideas? It has 124k miles

How long since the last transmission fluid & filter change?
If it's the torque converter clutches slipping, it might be past due for a transmission fluid and filter change and don't fall for the "flush" scam...install a new filter and refill with new fluid.

I’m sure its never been changed. I pulled the stick when I bought it. The fluid was brown, but it seemed to shift fine. I had planed on changing the filter and fluid but ran out of money. The fluid isn’t cheep. I didn’t know if that would help the problem, but its not going to hurt either.

A fluid & filter change fixed my '92 Bravada several years ago. It shuddered going up a grade in overdrive. I had it changed a couple days later and the shuddering has not returned...still got that SUV too.