Sometimes what happens is either the seal around the input shaft to the clutch pack or the 'o-ring' seal around the clutch apply piston cracks and leaks, resulting in no or slow/weak reverse engagement. Tranny works fine in forward?
A GOOD MB tranny shop will fix just the reverse clutch pack (the first one when disassembling the tranny. Should cost $130-150 parts and labor for the tranny fix if it's only the reverse clutch pack. No need to do the rest unless you want/expect another 140K miles.
Removal and reinstallation are a bear and cost?.