04 Silverado no second gear

Fluid Change has nothing at all to do with a fix
Nothing at all & on a worn trans can cause slipping
In your case I assume a shop gave it a try
The hard shifts tell you the valve body needs cleaning
I would first look at all body data related to the trans,whatever
your truck has TCM Module whatever
I would check all bulletins at a repair shop in their data base
I would have a mechanic ride with you to look at the trans
with the scan tool on it & watch the speed sensor & any
external sensors you may have for correct data
No also check the brake switch
Does it go into Lock-up in most gears (brake sw )

I have a 92 Crown Vic first and second works fine reverse works fine when it shifts in the third gear it goes to slipping and overdraft can you help me fix this problem thank you

This transmission is controlled by a computer, have that tested for fault codes first then move from there based on the findings.

Is it not shifting into OD? It should shift from first to second, then second to third, and third into fourth which is od. I'm not quit sure which forward gear you are meaning.

check color and smell of trans fluid, if not bright red and had a pugnant odor then the trans needs to come out and torn down, You can try a fuel filter that can cause a driveability problem

When you engage the "tow" button, all that does is stop it from going into overdrive and putting a strain on the clutch packs inside. The transmission shifts the same whether its towing or not.

It just holds 2nd forever. Is the 1-2 shift point high?

the kick down cable could me off but more than likly the trans in shot/ no good

Sounds as if 3rd gear is our of the transmission or it could be the shift selenoid.

you may have damaged the overdrive gear, a local transmission shop would be your best bet. for good troubleshoting and estimate.