The free wheel units screw to a hub which is solid with the roller shaft. They can each spin freely in one direction only and should lock in the opposite direction ie with the engine off and the drive engaged you should be able to push the machine forward but not pull it back. If both rollers can be locked with the above test I would suggest that your problem is roller slip on the ground and not a mechanical issue.
From memory the free wheel units have a small arrow etched into them to denote the direction of drive. Again, from memory I think that it is possible to assemble the ratchet units to the shaft the wrong way round resulting in no drive being transmitted to that roller.
The attached scan shows the general layout.

Quite possibly but depending upon how much you value the machine not a disaster and not a difficult job to do. Worst scenario is that the roller shaft drive sprocket is well and truly seized on the shaft and it has to come off to get the roller out of its carrier. Put a puller on it and it turns inside out, warm it up too much with the gas axe and you melt the plastic bearing carrier behind it. Carrier is cheaper than the sprocket but in a commercial situation if the sprocket didn’t yield so some gentle persuasion we would probably cut it off !
Try a bit of heat on the sprocket, careful use of the puller and some plus gas releasing fluid and with a bit of luck it will draw off.

Sometimes easier to weld them solid and then grip with 30″ Stillsons!