Caterpillar: Products>Machines>Track-Type Tractors
According to caterpillar web site, the current model D3 weighs either 17,185 (XL) or 17,842 lbs (LGP).
Parker Performance Trailers - Tandem Axle Gooseneck - Leading Trailer Manufacturer - Mt. Pleasant, Texas
Lets say, for example that your 28 ft. gooseneck dual tandem axle trailer weighs 6624 lbs and has gvwr of 24,000 lbs. Those numbers match the 28 ft. dual tandem axle Parker Performance trailer in the link above.
Add the 17,185 of the XL D3 to the 6624 of the trailer, and you are at 23,809 lbs, just under the gvwr of the trailer. However, if you have the LGP version of the D3 at 17,842 lbs, then the total weight of D3 plus trailer would be 24,466, slightly overloaded.
The other consideration is the tires on the truck. If they are overloaded, that would be illegal. I can't say what will be the load on the truck, because it depends upon how far back the axles are on the trailer, and how far forward or back you load the D3.
Let's say, for example, that there is 20% of the load on the gooseneck hitch, and your trailer loaded weight is 24,000 lbs. That puts 4,800 lbs onto the hitch. Let's say for example that you also have 3,500 lbs of the truck weight on the drive axle; so now you have about 8,300 lbs on the drive axle. Make sure that the tire load capacity is adequate for that much weight, and that you don't exceed the axle weight rating.
I would say that you are going to be pretty close; so I definitely agree with Nick that you must go to the scale and check to see that it is legal. If you are only slightly over on the load on the truck, you may be able to shift a little more to the trailer axles by moving the D3 back on the trailer, or if the trailer is overloaded, but you still have a little capacity on the truck, then move the D3 forward a little to take some weight off of the trailer and onto the truck.
I would check the gvwr and the tire ratings on the trailer first, because I am assuming that it is 24,000 gvwr. It could be a little more, or a little less.
In my state, California, the weight limit for moving farm equipment, even with a trailer licensed as a farm vehicle, is 26,000 lbs gcwr for the entire rig including the truck and the loaded trailer. If it is heavier than that, then the driver must have a class A license. I don't know if that is an issue with you or if you already have the class A license. Check with your state department of motor vehicles to see what are the limits in your state.
Since your D3 is a '70s model, it is possible that it weighs different than the new one listed on the www.cat.com
website, but I don't have that data available.