The flex coupler, what acts as a U-joint between the transmission output flange and the drive shaft.

Take a look at the drive shaft center support carrier bearing-bushing assembly. Is it secure? The bushing okay?

Else, engine operating problems. In lower gears the driver usually depresses the accelerator pedal more than when up-shifting and taking up final drive.

So, does flooring the accelerator pedal in fourth or fifth gear cause any problems?

What begins as intermittent failures, such as a failing sensor, corrosion at a wire harness ground point or wire harness connector, or other thingy, can cause "bucking" or "bogging" behaviors as you briefly describe here.

I did not think of these things earlier. Sorry about that.

Someone else may come along with useful questions and answers for you, versus what I put forth here.

cheers.

forgetful dud.