You have a bad unidirectional clutch in the transmission.
In drive, first gear is selected differently then in 1st. Selecting 1st manually engages clutch brake 3, which is not engaged in D. In D 1st uses the unidirectional to hold.

Any transmission shop can rebuild it and replace the uni clutch, however they may not be able to source one. I recently tried to locate one for a customer and could not, ended up replacing the whole trans with a re-man unit.

 

I new it was internal damage. When that happened, feel just like when the transfer jump to neutral.... Lol not a big deal. I already located one with 65k miles for 500