Shaking between 2nd and 3rd gear?

2000 Nissan Pathfinder, 150k miles, automatic
When I accelerate between 2nd and 3rd (from what it feels) the whole car shakes. No matter what speed I’m already at.
I recently had both CV axles replaced
Transmission fluid doesn’t have any flaky stuff or anything, it’s pink though. The check engine light is always on, but it starts to blink when it starts to shake between 2-3 gear. It’s been doing this for about two weeks, nothing has changed, average 15 miles a day.
No weird smells from the engine, leaking radiator fluid super slowly (was told to just keep filling it)
To start the car I have to press the gas slightly.
That’s everything I know is wrong with it. I just want the shaking to stop.
Does it sound like I need new transmission fluid or something?

Rather than a transmission issue, I suspect that this drivability problem is engine-based.
However, everyone is just guessing until you have the stored trouble codes “read” with a code reader.

If the CEL has “always been on” for an extended period of time, you should be prepared for multiple codes. In any event, continuing to drive it while the CEL is blinking is going to add a LOT to the potential repair costs. When that light blinks, it should be interpreted as “time for a tow”.