E320 5 speed - Won't shift into top 2 gears

[squealinneil]

My '93 E320 24 valve, petrol with 5 speed Automatic won't shift into 5th & possibly 4th. All the other gears are smooth & changes crisp, kickdown works. There are no noises - just won't change up! Does it harm the car/gearbox to drive it like this?
Anybody seen this prob. before & can tell me what's wrong? Is it going to be VERY expensive to fix?
Thanks .