04 nissan quest drives better in L than D

Our van has about 78.000 miles on it and has always kind of sucked, but it’s been sucking worse lately. There’s less power, everything seems more out of whack, the engine is noisier. The other day my wife tried putting the gear shift into L instead of D and it improved the power a bit and made the engine less whiny. Is this a clear indication it’s a transmission problem? The L is for Low Gear right? Last transmission fluid change was about 25,000 miles ago.

Is this a CVT transmission?

No, it’s not a clear indication of a transmission problem.

Based on how the question is worded, I am wondering if the OP knows what happens when a transmission is shifted into a lower gear. Just in case he/she is not familiar with this stuff, please be aware that downshifting the transmission causes the engine to run at higher RPMs.

When the engine runs at higher RPMs, you will get the impression of more power at certain speeds. However, your van’s top speed will be limited while in low gear and attempting to drive it at expressway speeds will lead to an early demise for the engine, due to “over-revving”. Even if you don’t drive it at expressway speeds, your gas mileage will be lower–possibly significantly lower–than if you just left the shift lever in the D position.

As to “making the engine less whiny”, this does not make any sense to me.
When the engine is running at higher RPMs as a result of staying in low gear, the engine noise will be louder than if it was driven in Drive.

I think that we need a lot more information:

What does “everything seems more out of whack” actually refer to?
Have the drop in power output, the “everything out of whack”, and engine noise problems been referred to a mechanic?
If so, what did he say? If not, …?
Aside from the trans fluid change 25k miles ago, what is the maintenance record (in detail) of this van over the past 3-4 years?
Are you actually experiencing any transmission problems (hard shifts, late shifts, slippage, higher RPMS than usual when in Drive, leaks)?
Have you checked the level, color, and odor of the trans fluid?
If so, what were your findings?

When an engine is running poorly, a lower gear will ease the strain on it. The solution is to find out why it is running poorly.

spark plugs and wires would be my first guess.