How can I get more power out of my 4.0?

Your axles are geared to the factory ratio which is only correct for the factory size tires. The 33" tires reduced the engine rpms at any given speed so its not developing as much power as it did with the factory size tires. Your rpms are so low from the factory axle gearing with the 33" tires your engine is lugging on the highway.

Your factory axles were geared to a 3.73 ratio and you need to regear them to 5.13 to get the engine rpms back up enough for the 33's. 5.13 will give appropriate highway cruising rpms and restore its drivability where it will again be fun to drive. Don't let some shop yahoo talk you out of 5.13, he doesn't know your 42RLE automatic transmission ratios and why 5.13 is the correct ratio for a TJ with your transmission and 33"tires.

Do what Jerry said. I have the same tires with a 4 banger and it's made the world of difference. Not only do I have more power in the upper range, it's also present in the lower range of that particular gear too.
 
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I have 4 speed auto, not 6 speed manual.
6 speed has overdrive of like 0.81 or something, which is really different from my 4 speed’s 0.69.
As for harmonics - when I drove my TJ with 4 speed, 31 inch and 4.56 gears 80MPH on highway (here in Houstonn area we have places with speed limit of 75MPH and really good roads), I was getting ~2850RPM. My driveshaft rotation speed was 2850/0.69=4130. Didn’t feel too much vibration. The engine was screaming, the transmission started feel hot, but very controllable and didn’t feel wrong.

You can do what you want. I’ve got two 42rle’s and a 6-speed. There are a number of threads documenting harmonic vibes with the 42rle leading the pack. Of course, those with problems seem to post while those that do not don’t. Consider @Shawn at Tom Wood's driveshaft calculator and article describing driveshaft rpm’s over 3000rpm’s. They aren’t able to balance for those accentuated drive shaft speeds. IMHO, the harmonic vibes issue is directly related to driveshaft spline engagement and balance. It’s your Jeep. Not here to tell people what to do.
 
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You can do what you want. I’ve got two 42rle’s and a 6-speed. There are a number of threads documenting harmonic vibes with the 42rle leading the pack. Of course, those with problems seem to post while those that do not don’t. Consider @Shawn at Tom Wood's driveshaft calculator and article describing driveshaft rpm’s over 3000rpm’s. They aren’t able to balance for those accentuated drive shaft speeds. IMHO, the harmonic vibes issue is directly related to driveshaft spline engagement and balance. It’s your Jeep. Not here to tell people what to do.

I am not saying that the vibes are not a problem. I had certain vibration before I replaced the U-Joints in the driveshaft. With new U-joints I don’t feel anything wrong.
What I am saying is that this is an obstacle that can be dealt with with balancing (even if it is only up to 3K RPM), correct pinion and driveshaft angles. I don’t think that giving up on regearing because of fear of harmonics is a good idea.
 
The engine's not screaming at only 2850 rpms, it's perfectly happy at those rpms. Mine were well over 3000 after converting my previous TJ from the AX-15 with 4.88 and 35's to the 32RH 3-speed automatic.
 
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