I'm getting engine hesitation right around 40mph/ 2900RPM. It may be the transmission but I'm not sure. Any ideas what's causing this and how I can fix it?

 

Are you 40-70% throttle shifting from 2-3? This is a common occurrence in the G when using manumatic. It sounds similar to a "thud."

 

Forgot to mention it's an automatic.

 

...

 

Very rarely do I use the manumatic and when I don't hear a thud, and there isn't any hesitation. I'm coming up on the 120k mi fluid change. Would you recommend that when I replace the trans fluid I add Lucas?

 

Hell no. Nissan Matic S or J only.

 

Have you changed your spark plugs yet?

"hesitation" could be from various things, but if you haven't changed spark plugs that would be my first guess.

It could also be from a sensor starting to fail, like the cam shaft phase sensors.