Prndl switch

Not if it did not have an issue at the time. Usually a rebuild is the seals and gaskets, bearings, steels and friction parts and any worn or damaged hard parts.

Is it usually covered under the transmission warranty?

Depends on the shop. Most only cover the parts they replaced. But a phone call to ask them would not hurt.

What is the switch doing? No start in park?

The check engine light came on and stayed on for 3 days but the truck isn't doing anything out of the ordinary.

Get it scanned and post the code(s) Many parts places will do that free. There are other issues that can turn the light on, like a loose connection, bad wire and more.

P0705 was the code

OK, That code can mean a bad switch, bad harness , or connections on the switch are bad.
I would look at the harness, make sure it isn't pinched or up against the exhaust or damaged. Then unplug it and check the pins for corrosion or damage.

Also check the switch alignment. The video below shows how it is adjusted. If the bolts were a bit loose or it wasn't exactly aligned it can set this code as well.

If the wiring looks good and the connector isn't damaged there are tests that can be done on the switch BUT they require a scan tool to watch the results. The switch itself is priced at much less than an average shops 1 hour diagnostic fee. So I would swap the switch. However you might want to call the place who did the rebuild. Tell them the issue and see if they offer any help.

Here is a video on replacing it without any special tools, it's an easy bolt on item.
https://www.youtube.com/watch?v=D8DAnlHCffI