You can't halt technological progress, but you can alter it to some extent and slow it down.
For example with your freeze of weapons I assume you want people to keep using line formations, slow firing weapons creating lots of smoke.
Alright, firstly muzzleloaders are going to be obsolete, because they were obsolete long before the civil war broke out. Why were they used? Because they were the easiest to produce, were on hand, breach loading tech admittably had some funks still to be worked out, the higher ups were worried soldiers would waste ammo if they could reload too quickly (ammo was expensive)... Still given the time passed in your world that stuff should be fixed.
Same with bullets, people had the idea of modern ammunition even if I do believe it was still mostly paper cartridges such as those used in the Chassepot and Dreyse gun. Still give it a bit more time and cased amunition will still be a thing.
How to keep some of that stuff around? Simply don't have people invent smokeless powders. Black powder forced a lot of limitations upon guns and tactics. It's less powerful, creates a lot of smoke and fouls the barrel.
Under these conditions firearm tech is essentially frozen. Gatling guns are the only rapid firing weapons that would be viable, semi-automatics are possible, but good luck seeing through the smoke after three shots. There's a theory this is one of the reasons people still used line formations in the civil war, since it was the only way to ensure hitting the enemy after the smoke screen gets created.
Edit: Oleg makes a good point about the effects of bullets and rifling. Even without smokeless powder, due to more rapid reloading and higher accuracy obtained through better ammunition and rifling, the line formation will still fall out of favour after a while.
You can sort of tell that the era of line formation was definitely over with the Chassepot already. However I would say it depends on who you're fighting. Colonial troops often used formations against the African tribes. I think it's highly likely that in your post-apocalyptic world there will be some groups that will be underdeveloped in terms of firearms and against whom these sort of tactics would still work. The thing that made line formations absolutely idiotic were machine pistols and machine guns, neither of which are possible with blackpowder (and even if they were imagine the smoke screen that would cause.)
That said the rest of my post should still hold up well.
With that horse cavalry also remains viable. The death of horse cavalry were machine guns and machine pistols both which won't be handy to utilize. That should keep the general military tactics and formations around.
The same could be done for brown powders and smokeless powders in the navy.
As for transportation, simplest thing I can think off is making oil hard to get. Coal engines are less efficient than combustion engines in boats and cars and airplanes and such. Cars were honestly pretty shitty in our world for a very long time and too expensive to get here in Europe until like the 60's. Lots of stuff happened between the 60's and the civil war. One or more things have probably been derailed due to 75% of the population being death. Though some armored cars will probably still find their way to the battlefield.
As for communication...simplest thing I can think off is simply making very difficult. So long as wireless communication isn't invented landlines are very easy to sabotage and if there's a lot of wild wasteland between city states a lot of things could sabotage telephone poles and such.
Beware that some stuff will alter histoy pretty drastic. For example I can see the air balloon maybe getting more popular in your world than it did in ours, since airplane development would be hindered. (There was a working steamplane though.)
You won't be able to keep it completely Civil war level technology, but you can get civil war-ish technology.