Another cheap fix is the output seals. Seals and new nut won't run you too much and are easy to install. If you can narrow it down to the output seals or from the shift point you're pretty safe and can use it as leverage on the price.
This was my front output seal. It would drip about a drop of gear oil if I let it sit for more than 24 hours. The dripping was coming from right behind where the front driveshaft bolted to the yoke. If it is this let me know and I'll pst up part numbers.