190 2.5D transmission noise ?

My wifes diesel 190 makes a annoying groan from the gearbox/driveshaft/diff ? when travelling at low speeds in higher gears. The noise seems to come from the rear of the gearbox but could be travelling along the driveshaft from the diff. The noise goes away once the load is reduced i.e. changing down to a lower gear [ 5 speed manual] or increasing roadspeed . There is no noise at high speed or high engine revs and no noise on the overrun. I have replaced both flexi discs and the centre bearing feels ok offthe car . The vibration damper is worn but my dealer tells me this is only available with a complete drive shaft !, I am not convinced that the damper will cure the noise. I have replaced all the subframe mounts and rear suspension links/ bushes. Do you think it is worth replacing the motor and trans mounts ?. Replacing the ultra expensive driveshaft ?, rebuilding the gearbox, or replacing the diff?. I have not driven another manual 190 diesel but I cannot believe that this noise can be normal, I have tried synthetic oils and additives with no improvement the noise is so unBenz like , please offer your advice p-lease Thanks Linds U.K.