Newbie needs trans help!

Hi all. I am new here. Bit of background. My 2nd 04 xterra and love them. The one I have now is a 04' V6 manual with 111,000 miles on her now. I have a huge issue I need help with. The trans is stuck in 3rd gear. I accelerated to to 3rd and tried to downshift and it just was stuck in 3rd nothing i could do. I pulled to saftey in a parkinglot. the cluth works, I can press the clutch and roll the truck. I can start it but cant get it out of 3rd. I replaced the clutch about 5000 miles ago, made sure the trans fluid was full and now this. I am at a loss. I'm not sure if it is a sycro, or if there is something else in there it could be. I am not afraid to do the work I just need my truck back on her feet. Any help would be awesome. Thanks

The bottom line here is the transmission will need to be removed and opened up. There are a couple of possibilities that could cause the problem.

A. A broken shift fork or roll pin that attches the fork to the striker rod

B. An interlock issue, an interlock problem could let the trans get stuck in 2 gears, in your case 3rd and 4th as they are both on the same hub.