Replacing the rear seals is not particularly difficult. The lower end of the strut can be detached, then, with the wheel hub volt removed, you should be able to pull the brake assembly outward. A few taps should free the drive shaft spline from the wheel hub. You may need a helping hand to hold the brake/strut assembly outward while another set of hands manipulates the drive shaft free. There is some play in the universal joint so you sound be able to extract the drive shaft. Pull on the drive shaft to remove it from the transaxle - note: you will spill transmission fluid so put something under to catch the fluid. The seals pop out without too much force and need to be pressed back into place. Be careful not to damage the mating surfaces when pulling out the old seal. Installation is the reverse.
Evolution, your Smart’s best friend, has excellent write ups for removing the drive shaft and seals with pictures.
One tip - if you are going to remove the drive shafts- do yourself a favor and replace your reluctor rings at the same time - it will save you the aggravation of having to remove the shafts again to replace them!