edward is correct, you cannot go by 'feel' , you have to use pressure tests to know what is going on inside these transmissions.
flushing the transmission case with diesel fuel is not going to get you a good cleaning so you will have debris floating around. It would take several fluid and filter changes after running the lift to get the fluid clean OR pulling the transmission and disassembling it and cleaning it which would be during a rebuild generally.

also there is a screen nobody thinks about that is in the control valve area which is what in training we were instructed to call the 'last chance filter'. It is a small screen in the ports between the control valve and the transmission body, it is not shown in the manual either, it is part of the assembly and if it gets clogged that will throw pressures off and could keep the truck from pulling. Also check the strainer located in a plug under the solenoid valve chamber.

If you want to see the condition of the plates you can pull the control valve and take a physical look at the plates if you like, you can usually get an idea of what condition they are in that way.

I think you'll find if you have that much debris floating around inside the unit these plates will be worn out.

I suggestion would to be start looking for a reman transmission unless your in the mood to rebuild one.