I had the same problem with my F250 7.3 lately. I was not having any issues, but decided to get a trans service just because of mileage. During the service, I saw the trans magnet and there was almost nothing on it. Trans was in great shape and no concerns noted. After the service, it was running fine in drive, but in reverse the RPMs would drop real low. If I kept my foot on the brake when putting in reverse, it would drop RPM, but engage and then seem okay. If I took foot off the brake before it engaged, it would almost always stall out. Took it to Ford. Their tech sat in it, put it in reverse once, then said I needed to replace the Torque Converter. No other diagnostics. That will be $1200 thank you. I don't think so. I took it back to the trans service place and asked them to look into it. They took the pan back off and found out they installed the wrong trans filter
. It was similar, but off just enough to change pressures when shifting. Don't know if they ordered wrong one, or parts house dropped off wrong one. Either way, got the correct filter put in and it runs like never before. No more low RPMs or stalling out.
It is a cheap thing to check on before a new trans, a torque converter, etc.