I don't think suspension would be worth doing Norm ,The quality of the cut is generally optimized for a fixed chassis. Changing this could affect the mower's cutting performance and result in uneven grass cutting.It would be a lot more work to get the cutting deck independent from the mower chassis so suspension won't effect the cutting height when driving over bumps.
It's been over 30 years since I needed to make a locked diff for a mower and it's still going strong even after towing 600 pounds up a 10 to 15 degree slope at least 20 times.(diff has no spider gears) the side gears are welded to the axles.
