1999 3.0
140K
4R44E (4-speed)

I don't have direct access to the transmission in question, because I'm only considering a purchase of the truck above, but it seems like a good deal except for the ONLY issue the current owner says it has. It appears 1st gear doesn't work/engage if the lever is in the "D" position (revs up, but goes nowhere), but works fine if it's in the "1" position. All other gears work including reverse. There is no slipping, no hard shifts, no noises or smells. For all intents and purposes, everything works perfectly except it won't start from a stop in Drive.

My question is...what are the most likely causes of this scenario?

Thank you.