V62,
Yes the 682-782-1800-2000 series from 85-92 era should all work - Cub used the same hydros in them. If you do a search on line you can find this out.. THe earlier verisons than the 1800 series might have a cast rear end which is preferred. Also if your 1872 doesnt have the hitch plate (hitch is attached to the AL rear end) I strongly suggest the rear hitch plate that connects to the frame rather than the rear transaxle.
As far as getting a hydro working assume this is your 1872, so as suggested, clean Hytran fluid, Cub filter (some 3rd party filters will block flow) so its cheap insurance using a OEM.
The other things are the relief valves on top of the hydro usually they leak first but if stuck can cause issues.
You really dont provide enough info to reallly help your situation.. Please clarify what you are really trying to attain!