2002 Nissan Pathfinder Transmission issue

Two years on an oil change? Don't expect to get 200,000 miles from the engine.

Engine shaking is due to a misfire. There are dozens of possible causes, but the most common is worn spark plugs and wires. Without any test results to analyze, that would be a good place to start.

OHHH! NOW I feel better. Sorry for not figuring that out.

Here's probably the most important clue:

"Then the shaking continues if you shift from reverse to neutral."

The transmission is out of the picture when you're in neutral, so until we know differently, lets forget about looking at that. What IS changing is the engine's position. It is normal for it to rock when shifting directions. One of the engine mounts is responsible for controlling how much it can rock, and how much it can move during acceleration. Based on the way you can get it to shake and stop, it sounds like you have a collapsed or deteriorated engine mount. All of them are two-piece metal brackets with rubber isolators in between. When that rubber tears or crumbles, the two metal parts can touch each other. That will transmit normal engine vibration into the body where you will feel it, and it can be real irritating.

If I'm right, you may be able to see the excessive engine rocking by putting the transmission in "park", then open the hood and watch how much it rocks when you push and pull on the car by hand. If it's really bad, you'll even hear the metallic clunk of the two brackets hitting each other.

Another pair of engine mounts hold up each end of the engine / transmission assembly. A collapsed mount will let that end drop down. You may get a vibration in the steering wheel when you accelerate, but mostly you'll just get that irritating vibration in the car again from the two metal parts touching each other. On some cars it's real easy to see when one side of the engine is sitting low. Some are hard to tell except by mechanics who are real familiar with that model and engine size.

Nope. The engine turns a fan that spins transmission fluid. That spinning transmission fluid forces another fan to turn. That one is hooked to the transmission. All of that is placed inside what looks like a giant metal donut, and that is the torque converter. The slippage between those two fans is what allows the engine to idle at 800 rpm in gear while the car is not moving.

It's also that slippage that reduces fuel mileage compared to a manual transmission. To make up for that, Chrysler introduced a lockup clutch in their torque converters in 1975. It was hydraulically-controlled based on road speed and only engaged above about 40 mph in third gear. Today they're computer-controlled through a solenoid that turns fluid flow on and off. The entire system is not in the picture in reverse or until road speed is high enough. The only time the lockup clutch will cause a vibration is during the couple of seconds it takes to engage when the wrong transmission fluid has been used. That doesn't hurt anything. It's just mildly irritating.