Transmission would not engage in Drive

My 2005 Tacoma has 183k miles. It was parked in our driveway, uninsured for the last 3 months.

This morning I decided to start her up. The engine started up fine, but after I shifted to Drive, the truck wouldn't move.
The engine would rev as if it were in neutral.
Over the next few minutes I tried shifting between park and drive. Eventually, the I heard a loud 'thunk' when I was in Drive and gave it more gas. After that, the transmission seemed engaged and the truck was able to drive normally.

I drove the truck around the neighborhood and tested park, reverse, drive, etc. All behaved perfectly fine, albeit a bit rough from age.

Has anyone encountered something like this before? If so, what was the diagnosis? Did you find a solution?
I'm wondering if this is a sign of major transmission problems.

 

Most likely dirty fluid and a solenoid was stuck. I would begin a series of transmission pan drains and a filter change.https://www.tacomaworld.com/threads/how-to-drain-refill-automatic-transmission.63851/

 

Not sure where you live but cold temperature combined with a lack of use could be to blame. I’d definitely start by checking the fluid level and then go from there as stated above.

 

After sitting for long periods fluid can drain away from valve body causing sticky valves (mentioned above) and delayed engagement.

Like others said
changing fluid sure wouldn't hurt