I have a 2004 bobcat 331, and in low speed mode one of the tracks is way slower and lacks the power like the other side, but in high speed mode both tracks work just fine. At first I thought maybe final drive problem but since it works great in high speed mode it seems like it may be something else.
Good day Possible weak motor if the problem is forward's as well as reverse, the weak motor will need less power when the stronger motor to carry the machine. If you dead end the motor lines and compare pressure readings with low & high revolutions, it will give a good indication as to where the problem is at. Kind regards Uffex