Would require way more info that you'll be able to give anyone online to help you even if they knew how. 2 shops have had it in person amd are still scratching their heads, so...your limited description not going to get any definite answers over the internet. Sorry for your troubles.

Hopefully its something simple and cheap. When you say won't shift into drive you mean you physically cant get it to shift to drive or you mean you shift to drive and it doesnt actually engage the drivetrain and move? Does shifting to other gears have any effect?

You don't have the truck at this point so it isn't like anyone could wall you through any sort of troubleshooting anyways even if you were able... And assuming you were that able, it wouldn't be at a trans shop/dealership. No offense intended... Just come from years of professional wrenching and repair facility service work and know how frustrated people can get hemselves trying to diagnose blindly over the internet and listen to random clueless strangers because they lack basic fundamental knowledge and trouble shooting skills to even determine is other peoples' anecdotal experience is relevant in any way.

I hope they find it quickly. Intermittent issues are always the roughest though, hipe you get a good tech that gets it beat for you quickly. Hang in there!