I agree with Steve's suggestion that you do a bit of diagnosing to see whether the levers move freely with the valve spools disconnected. Unless things are locked in the detent position (all the way forward on the inner lever), I strongly suspect the lever bushings, not the valve.
If you do need to do something with the valve, I'd suggest disassembling and cleaning the one you have. You'll need the TM for your tractor.
I have TM1590 for the 318/420, and the overhaul procedure talks about five versions and three basic configurations for the 2-spool type. Presumably these are interchangeable as assemblies. This may apply to your tractor as well.
You'll want a good 11/16 tubing wrench for disconnecting the hydraulic lines.
There is plenty inside besides the spools, so you will want to take care on disassembly. Put down a sheet of newspaper on your bench and lay the parts out in exploded view order as you remove them.
Deere sells overhaul kits for these units. Those include all the soft parts and expendables. Wash all the hard parts. A good overnight soak in lacquer thinner generally cleans things up "like new." You can check for free motion of the sliding hard parts in the valve body after washing. I'd consider polishing parts as a last resort emergency repair. Generally, this type of valve is free after a good washing, and a bit of dry-lapping by just working the parts back and forth will assure that they are free.
On reassembly, use vaseline for assembly "stickum" to hold pieces in place, and oil everything with hydraulic oil. Cleanliness is an absolute must when assembling and reinstalling.
Wash hands, and don't oil until just before installing parts.