Leeellen

Hi I am new to this site so please be gentle.....I have a 03 vogue 30ltr diesal and the trans failsafe program has appeared on the dash and the car sticks in one gear I assume 4th....this remains like this till you switch off but comes back on as you start off again.....It's been to a local garage which said it was due to low oil in the transfer box due to a leak...so leak was repaired and shifter was bench tested to make sure it was working ok which they say it was.....£300 later drove car home and fault came back so I assume it wasn't that so £300 lighter and no closer to fixing problem could anyone help as garage is saying they don't know what it could be now.. Thanks in advance