There is an upper drive shaft housing and a lower drive shaft housing around the drive shaft. You state that you continually must remove the lower shaft or edger unit, adjust, and re-install. Unless there is excessive wear or something is broken, there are at least 3 possible places for the drive shaft to be "too long." These are at the place where the upper shaft fits into the engine unit, at the coupler that joins the upper and lower shafts, and at the bottom where the edger head fits onto the lower shaft. To make sure that the pieces go back together properly  mark each part of the shaft, coupler, etc. so that you get proper alignment when putting it back together. For the upper drive shaft, remove or loosen the coupler where the upper shaft connects to the lower shaft. Then loosen bolts where the upper shaft connects to the engine unit. Do a couple of short twists and turns with the upper shaft to make sure it is slightly loose  and with a little force, push the upper shaft housing into the engine unit to seat it properly. Re-install the coupler, making sure that it also is seated as far onto the upper shaft as is required. At the bottom where the lower shaft fits into the edger unit  loosen any holding screws and make sure that the edger head is pushed far enough onto the lower shaft, then re-fasten the screws.