Model and serial tag should be on the right side of the frame below the engine compartment side cover. If the tag isn't there you could have a 3205 or a 3208 as both had differential lock, aluminum differential, and the Kawasaki 20hp engine. Based on the pics I would say you need to tear into the differential and replace at least the seals on the inner axle bearing blocks which is why you are loosing fluid, but I would also replace both seals on the outer bearing blocks as well while you had it apart. See Oak's link posted above. The attached link is the spec sheets for the 3000 series from 1998-2003. I don't think you'll be able to tell from it whether you a 3205 or 3208 but it has good info.
https://www.cubcadetpartsnmore.com/c...et-spec-sheets