So I figured my 3rd gear was dying and I would just have to swap the whole transmission but someone mentioned putting in a shift kit. I always thought a shift kit was some performance mod but I guess its more of a repair thing? So how's this work now?


I put in that and new gaskets and a filter and see what happens?

 

Automatics have just one gear set, good animation of that seen here: www.youtube.com/watch?v=Y1zbE21Pzl0

What really makes an automatic work are the clutches and Bands(brakes) and the PRESSURE it takes to engage and release the clutches and brakes to get the correct ratio("gear")

Reverse requires the highest pressure, 150+PSI, so when there is a pressure loss issue Reverse being slow to engage or not engaging is the first sign.
Torque converter drives the transmissions main pump(front pump) so increasing engine RPMs increases transmissions internal pressure.

For the ratios to change certain parts have to be disengaged(clutched) and STOPPED(braked) from spinning, this is all done by applying PRESSURE
When Bands are adjusted its to make sure the BRAKES are able to stopped a section from spinning.
If pressure is low then clutches can rub, same as "riding the clutch" in a manual trans, so clutches burn out, and brakes can't hold, which burns them out as well.
And this rubbing also generates lots of extra heat, which kills automatics.

So PRESSURE is the main thing that keeps an automatic working.
The Valve Body routes the pressure to where it is needed, the solenoids control that routing.
If valve body seals and gaskets are leaking internally then you have loss of pressure, and that causes problems.

So replacing valve body gaskets and seals can solve many automatic transmission problems, but NOT ALL.

The clutches and brakes(bands) inside an automatic wear out just like brake pads do on wheels or clutches do in manuals, just standard wear and tear.
Slipping can be a sign of that but pressure loss causes that as well, but in any case slipping is HEAT building up, and that for sure causes higher wear on clutches and brakes


Will the new valve body solve your problem, don't know, but if it doesn't and you need a full rebuild at least you will have new valve body, so not a waste of money, just the time to do it