Could be transmission clutch like SJ says and it probably is, but could also be steering clutches, though it would have to be both at once. If the steering clutches are in need of adjustment, there will be very little free travel when you pull the levers, should have around 3 inches at the bottom of the grip in the levers. If it's the transmission clutch, you will have no snap as you pull it in and push it back out. It doesn't have to be a real heavy pull, I believe the D2 calls for something like 35 pounds pull to snap it in.
Later dry clutches were adjustable from the top through a larger top cover opening as were the oil clutches if you have one of those. The older dry clutches with the smaller access covers were best adjusted from beneath so that the lock bolt could be tightened securely. It would be a good thing to adjust it before it winds up stuck in a spot where you can't get underneath and have to have spagheti arms to reach in there.
You might also want to make sure if it's a dry type, it isn't being oiled as the tractor runs. There will be a drain plug in the clutch housing to drain any water and oil that have accumulated in the clutch housing. Dry clutches have a slinger to keep any oil from the rear main bearing from getting onto the clutch disk itself, but it will accumulate in the housing over time.
D2-5J's, D6-9U's, D318 and D333 power units, 12E-99E grader, 922B & 944A wheel loaders, D330C generator set, DW20 water tanker and a bunch of Jersey cows to take care of in my spare time:D
