If the shop did use the wrong fluid, Can I flush and totally purge the system and refill with ATF+4 to solve the problem?

 

I do not know if the wrong fluid would cause so much slipping, rather harsh shifts, shuddering and excessive wear.

Some places have machines which will exchange all the fluid, but then you run into the same problem, they most likely are not going to use ATF+4. Several years ago, I paid 280$ for a dodge dealer to do it, when a tranny rebuild shop's universal bulk fluid was causing bad torque converter clutch chatter at 47 to 60mph light accelleration

There is another method where you take the line off the radiator insert it in a 5 gallon bucket,start the engine, put it in neutral, and try to pour new fluid down the drain hole at the same rate the tranny pumps out the old fluid.

I have never done this, and am not sure that there is not some mixing of new and old fluid in the pan.

I thinks you have larger issues than just the wrong fluid.

 

Yes, the wrong fluid WILL cause slippage, harsh shifting and very bad shuddering. Been there and done that with a 1996 Dodge Caravan after a tranny shop service. They put in the dumb GM stuff.

Good advice. Done this a few times already with various vehicles.

Yes, after the flushing, drop the pan, replace the screen and filter is a MUST, because that other fluid just coats like snot. Not a hard job to do either. It's so easy that a bunny with a pancake on it's head can do this service.