1984 300TDT w/Automatic. Shifted into Reverse, and backed out of the drive. There was a "clunk" when the tranny engaged. Tried to shift into Drive, but nothing. I now have no forward or reverse. I checked the linkage, and have the correct motion through all the linkage down to the tranny. I suspect something inside gave out. How serious is this, and is there any chance that a fairly mechanically minded person can resolve this, or is it off to the transmission shop?