Transmission Slips from 2-3. 1st - 2nd is good, 3-4 is good

If it is more of a delayed shift, I would start with the 2-3 shift solenoid. If it is actually slipping (can make it slip after shift is complete) then I would say you have a worn clutch pack. Since the fluid is clean, my first guess would be the shift solenoid.

Honestly, if you can get it to slip once it is in gear, it sounds like the clutches are spent or there is really low trans fluid pressure which I doubt. As far as this type of rebuild, it isn't that bad to do. Basically, it is a soft parts overhaul, clutches, seals, gaskets. Yes, I would recommend getting a good repair manual. Make sure you have a sliding dumbell to get the front pump off. Also, when you put the clutch packs back together, MAKE SURE the clearance between them is within spec. If you don't and they are too tight, you will end up with what feels like a double shift. If there is too much clearance, it will have a delayed soft shift and will wear the new discs out quickly. Inspect everything when you have it apart for stress cracks. Also, make sure to replace all thrust washers that will come with the kit. Based on what you have done in the past, take your time, get the manual, and it sounds like you will be fine. If you run into trouble, let me know.