most likely the tranny filter has dropped in to the pan,drop pan,if so this creates a pressure differential in the valve body where it shouldnt,common in older fords,will affect both forward and reverse,usually reverse first.
How do you answer a question about a vehicle when you don't know what it is? No engine size, not the right info.
85% of the people have no idea what we are talking about even IF we do get enough input to give advice anyway.
Oh, and the other BS from a DA, so called mechanic, whom i don't think has ever had a job. I give up! GOOD LUCK.
IAC valve stuck open. very common!! you can try tapping on the IAC with wrench or something while truck is at idle. have someone put thetruck into gear. if it stays running,the IAC was stuck. replace it. easy fix. alot easier than torque converter