I'm thinking it is a Dash 3 series machine and there is a lot more to the situation than the swivel. The system is an open center load sensing. There are cut out functions on the pump that could be down, the main reliefs could be low, the cut out cancel function may not be working or the TVC spool in the pump could be stuck.
With all that said, check the cheap to fix stuff first. You could have pilot controlled pedals but most likely have the old cable setups. Make sure the travel spools are getting stroked all the way when moving the pedals. Can you check main pressures? If so that needs to be done and they need to be set even. You could check case drain flows on the travel motors. You block the sprocket from turning and take off the case drain hose. Cap it off and run another hose from the fitting on the motor into a bucket. You should only see a trickle going into the bucket when you turn the track and it doesn't move. I forgot to ask if the weak travel is on both sides. If so, the case drain is a good test. If the motor is good then it should be time to check the swivel. Usually you will find seal material in the pump suction strainer for a good clue but that isn't an absolute test. The swivel is easy to get apart. The line drawings from lantraxco has most of what you need. The seals go in the barrel and you can take it off and leave the center section in the machine.
That should keep you busy for a while. Let us know what you find.
