Did you check your hydrostatic bypass lever? What you are explaining could only be that or possibly something with the linkage between the drive pedals and the transmission. With it off you could lay underneath and use your hand to move the drive pedals to see if the linkages are still connected to the transmissions and that they are actually moving the lever on the hydrostatic. If all connections are good and the bypass is not engages, then check that the brake is releasing. If I remember correctly, the brake on this might also take some tension off of the drive belt when depressed. If the drive belt is under full tension, then you may have damaged something inside of the transmission. The LT155 should have a Tuff Torq K51B, which are sealed units but can be repaired if you are daring. Otherwise they can be found used for a couple hundred bucks online. Good Luck!