I had my transmission serviced with new filters and fluid. The mechanic also replaced a solenoid to resolve a code. Now the transmission freewheels for a period of time (2-5 sec or so) after shifting from reverse to drive. Does it hot and cold, no matter how long the truck has been running.
He's quoted me $4K (about half the value of the truck) to fix it.

Is there any inside knowledge to be had about the described symptom and transmission, which I believe is the 545RFE?

It worked fine before the service (besides the code)? I know some guys have had the push on filter fall off and then have nothing in any gear, but I'd suspect your "new" solenoid pack to be the problem. Is it new Mopar OEM stuff or a rebuilt jobber?

did they perform a quick learn after replacement of solenoid pack? if not that could be an issue.

Almost sounds like he didn't replace the seal where the sump filter pushes in. Some filters come with the new seal, cheaper ones don't. Did he replace both filters?

This I agree with as I have seen many not get the sump filter all the way in and it sucks in air. And did he replace both filters ? My question is did it do this before he worked on it and how did it act when it had the code in it before he replaced the solenoid pack ? Ron

The only symptom I had before the service was occasional sluggish starts from a stop.
I asked him to replace both filters. He mentioned something about learning but I'm not up on the details about that. I'm not sure about the solenoid, I'll have to check my receipt.
Is this something I can reasonably drop the pan to do myself? I have run out of trust with this mechanic.

He most likely didn't get the filter tube pushed through the seal, I'd say he just got it up against it and then installed the bolt to hold it up. There is only one small bolt that holds the filter in place. Drop the pan (messy job) and remove the bolt holding the sump filter, it will drop down as you remove it. If he correctly installed the filter you should have to pull and twist the filter to get it to come down out from the seal, if it drops right out it wasn't installed into the seal. If it were mine I'd buy both new filters from Napa with the seal and replace them both while you're in there.

The receipt doesn't specify the solenoid, but the filters are Pro-King.

Another point to remember on the RFE transmission is that the sump filter is specific to the driveline type 2wd vs 4wd. IDK if its possible to get the wrong filter and the pan will still install.