Your reverse clutches are worn out when you get that feeling. If it just slipped it might only be a seal. its the same difference anyway as it has to come apart to fix it and you will replace both parts. In your case when it feels like its in two gears you have gotten the whole pack so loose that some of the discs are coming unsplined and jam. You will probably need the steel discs also. They aren't real expensive. The real damage will be done after they get metal to metal the pieces go everywhere and easily foul the valve body.
Bosch Master, ASE Master, L1
33 years MB technician