there are a couple of t5 rebuild videos out there... hanlonmotorsports had one of them. YOu can get a free manual from tremec as a download for the t5. they're simple as cake. All you need are snap ring plyers, dial indicator and a press. If you don't want to buy a press..you could likely take it to a local shop and pay them a couple of bucks. 90% anyhow can be assembled and disassembled without a press.
sounds like an internal issue to me. Pull the trans down and yank the lid and tail shaft. Inspect the forks. I would think that your forks are fine, or you'd have an issue going into 5th too.
It's likely an issue with the reverse lever/shift rail inside.
However, before you pull anything, get a torquex bit on the bolt on the dr side top of the trans. This is the reverse sensing switch for the reverse lights. On occasion, the little ball in that switch will seize, and keep you from going into reverse. You can pull it out and be safe to try getting into reverse with it running. Just don't drive it like that, or you'll lose fluid.
It very well could be the reverse ball/switch