Hmm.. Sounds like what happened to mine a few years ago. Same symptoms - no forward drive but OK in reverse.
One way to check if it's terminal is run the engine and go through the gears manually and if the cluster indicator says it won't select or stay in 3rd (or it could be 4th I can't remember) then you can be sure it's one of the drum packs broken.
It cost me £2100 here in the UK 3 years ago but mine is a 4.4 so the trans is more common than yours.
Check out Zero to 60 on YouTube. He is / was a BMW Australian guy and had a red 4.6 that they rebuilt themselves a few years ago. Interesting.