Common problem with the AOD w/120,000 miles.
You burned out the overdrive band within the tranny.
This can be replaced, but you have to drop the trans to do it and if that band is toast, the others will be mostly effected.
So the bad news is, time for a rebuild or a replacement.
Start saving up your money. A stock rebuild will be around $1300 depending on where you go, and a performance buildup or new replacement is gonna rock you $2500.
You might consider a 5spd conversion and get those parts used and be somewhere around the stock rebuild price with labor.
Good Luck.