Hello and welcome back to JustAnswer!
The first thing you'll need to do is have the codes read that are stored in the transmission control module. Any time the trans goes into limp in the TCM will store fault codes to aid in diagnosis. if you don't have access to a code reader or scan tool, almost all parts stores will read codes for free in the parking lot.
Since you've replaced the relay and the speed sensors the next most likely cause for your problem would be the solenoid pack mounted to the top of the valve body. This contains all shift solenoids and pressure switches for the various clutch circuits.
Other possibilities would be the TCM itself, wiring from the TCM to the trans, or internal trans. If you don't have any slipping an internal trans problem would be less likely than an electrical one.
The first thing you should do before replacing any more parts is see what codes are stored in the TCM. You don't want to replace the solenoid pack if you don't have to! if there are solenoid or pressure switch codes set the problem could be wiring or the TCM but is more likely the solenoid pack itself.
The solenoid pack is mounted on top of the valve body. The valve body is the large aluminum piece inside the trans with dozens of fluid passages that routes the fluid to the correct spots. To replace the solenoid pack the valve body has to be dropped. This would involve removing the pan, shift linkage, and then dropping the valve body. Then you would be able to remove the screws holding the solenoid pack to the valve body and separate it.
I did, and I thank you! Good luck with your car!
What does it do when you take off from a stop? Does it upshift into second and then freewheel when it should shift into fourth?
How was the fluid when you had the valve body out? Did it smell burnt at all?
How much metal was in the pan?
Is the problem intermittent or consistent?
Sorry for the delay, I'm a full time tech and I'm at work this afternoon.
What happens when you take off from a stop? Does it hit second gear hard and stay there? Or does it shift into second, then try to shift into third and freewheel before it goes into limp-in?
Does the speedometer always work ok?
Were you able to watch the input speed sensor signal on a scan tool to make sure it's there?
It would be beneficial if you could drive the car with the scan tool on it again and watch the input and output speed sensors. Make sure neither one of them is dropping out. You aren't setting any speed sensors codes but sometimes you can have speed sensor problems without setting their dedicated codes.
If this scan tool has the capability of doing clutch tests try all of the clutch tests to and see what you get.
Did you hold the brake while doing the clutch tests? I'm assuming the clutch test on the Snap On scanner is going to be the same as the factory scan tool. You put it into the clutch test while holding the brake and open the throttle. If any speed signal is shown on the input speed sensor while doing the clutch test it indicates a slipping clutch.
It sounds like the input speed sensor is working all the time isn't it? If the input speed sensor is working all the time then you have an internal trans problem causing this to happen. The gear ratio codes you had set all have to do with the input clutches - you had 3rd, 4th and reverse ratio codes. The reverse clutch and the overdrive clutch (used for third and fourth) both use some of the same seals to apply. You may have some seal leakage, clutch damage, or even a problem in the planetary geartrain.