The 4 wheel drive quit working on my 3510, too. I recently had the hydraulic pump rebuilt, and wondered if the 4x4 could be affected by the hydraulic system(?)
I've thought the 4 wheel drive shift lever has always been a little weird to operate since I bought the tractor [used] a couple years ago. The lever only travels about half way through the lever slot opening, and I can tell there's a slight 'clunk' when it goes into 4x4. When I first got it, I thought the lever needed to be all the way down and when I put some pressure on the lever, I could hear what sounds like gears grinding, like it's trying to go in gear with gears still turning. So I thought the 4x4 wasn't engaged. But later I found the 4x4 worked fine, so the lever isn't all the way forward in the slot when it's engaged.
Anyway, the 4 wheel drive is definitely not working now. Could the shop have done something when they rebuilt the hydraulic pump?
- djb