My 23 spline A body 833 won't shift out of 2nd gear under high RPM. Soon as the RPM drops to around 2500, it will shift like normal. This happens around 5000-5500. Any ideas. It's not the driveshaft.

What about shifting in other gears? Problem just started or always like this? Pilot bearing or bushing is my first guess. Ball detent side cover?

Just put the transmission in. Like this from day one. New pilot bushing and ball detent cover.

New transmission? How did the stop ring/synchronizer look for that gear? And then bellhousing runout and parallelism?

I've had that problem, is was cause by the clutch not having enough travel on the throw out bearing not disengaging the clutch plate completely
If your clutch disc is a street unit with a wave (marcel spring) spring in the middle of the clutch disc you need to adjust the clutch linkage so you have at least .060 air gap between the pressure plate or the flywheel and the disc with the clutch pedal depressed like you shift it at.
If it is a race disc you can run them a little bit tighter, say minimum of .040 air gap IHTHs

I’m with cab. Sounds like clutch adjustment.

Did you dial in the bell housing? Is the front bearing collar the same size as the bell housing register?

I was thinking one of the bearings on the main shaft, but I will look at the clutch gap first.
Thanks

Hopefully clutch adjustment, otherwise What he said ↑

Did you adjust the shifter?

What oil are you using? When I ran synthetic in mine, would not shift over 4 unless I dropped the rpm. Was told that I needed to run the old 140wt oil as the synchro rings were designed for that. Took care of the problem. Later on I took it further by having it slick shifted.

I was just reading thru my 69 shop manual and saw the recommended oil at 140 wt , I always ran 75-90w which they says to use in cold weather and auto trans for extreme cold , I'd never run auto trans though , I can tell the 4speeds that had auto trans by the amount of wear on the cluster pin and reverse gear shafts.

To the op , what oil are you using ? you need something that is rated for brass , castrol syncomesh , if it's still available, or Passon has a oil he sells.

OP's issue sounds like a misalignment (maybe inside the trans) or clutch issue.
Over the 33 years I had my Hemi (18-spline) car, I had different lube in the trans: the OEM stuff, 75/90, synthetic, ATF.
Only difference among them for shifting was the lighter weights shifted a bit easier, especially cold.
Powershifted that thing many times from 6,000 - 7,000 RPM; it was never stuck in the lower gear, though I could miss the next gear if I wasn't focused.
No issues ever in normal driving.
We did fuss over the crank/bell/input alignment.

My 23-splines are a bit easier to shift, though I don't beat on them.
I have had 1 that a previous rebuilder screwed up a snap-ring or two.
Initially hard to get Rev or 1st, IIRC, and quickly - like 2 miles - degenerated into horrible noises and reluctant to shift at all.

Won't shift out of 2nd or won't shift into 3rd? Make sure you have lube for BRASS synchro rings. Not synthetic and not rearend lube. Many of the lubes in the stores are for modern transmissions and are incompatible with the brass rings in 833s. The synchro ring has to be able to grab the cone on the gear. If the inside of the synchro ring is worn out it will not grab the cone no matter what lube is used.

Many NP833 were sold with straight ATF in them when the cars were being sold by dealers in cold winter weather states do to hard shifting complaints by the early customers who bought cars in 1964 and 1965, I ran straight ATF in all of my 4 speed Mopar's while living in the Mojave desert in SO CA for over 40 yrs. with no bad results or failures

i could never shift for sh!t at high rpms, no matter the shifter, transmission, or fluid used, so i just wimped out and have used automatics to this day...........
i really like pushbutton shifters. i have one planned for the A518 going into my "eternal project", my 1933 dodge "humpback" panel truck.
i figure if imperial services can figure out how to do that , i can too.

I can easily see the difference between a trans that had ATF and one that didn't in the depth of the wear groove in the ATF trans on the cluster pin and reverse gear shafts .

Pulled the cover off the 4 speed and one of the shift struts is missing on 1-2 synchronizer. Guess that explains the no
shift under high RPM. Pulled the tranny out and didn't see it in the bottom of the case by the magnet? ? ? The retainer
springs for the struts were wobbly looking and bent.

Are the shift struts soft enough to get ground up and not hurt the gears?

only a visual inspection can determine that.
i would be concerned about the ground up bits getting in the bearings or bushings, or both.