To try and fix transmission leaks. I've created a little checklist. This only pertains to swing-axle type transmissions, I don't have too much info on IRS, but I'd think they'd be very close, and possibly less prone to leakage.
1. Check and make sure the vent hole is open, the gasket that goes between the intermediate housing and nosecone might be blocking the vent hole. On our self-made gasket it was, I opened the hole and used a little sealant (Indian Head) on the gasket.
2. Make sure side cover O-rings are in place. Gasket on side cover was also self-made, a little sealant again here.
3. Axle tube retainer plates, this frequently go out of flat, from over-torquination (yes I know it's not a proper word), use a the ball end of a ball-peen hammer against it and smack it flat (use the ball-peen hammer like a drift pin). I tried my best here. Also coat every single layer of paper gasket used to set axle tube play with sealant. I also siliconed (here is the only place where silicone sealant should be used) the plate to the side cover.
4. Rubber boots, put them with the seam in the 2 or O'clock position, The clamps that came with our set sucks ass, it'd pull apart with enough torque. Put sealant between the seams before screwing them together, don't tighten too much, just enough to pull the 2 halfs together. You also want to position the clamp so that when you tighten, it pulls the halfs together.
5. Leaking at wheel bearing area. Make sure you have all the bits as shown in the above diagram, oil and install the O rings, after you have the inner spacer and wheel bearing in. Install the gasket with a little sealant onto the seal retainer. make sure the backing plate (even on the back side) is clean, if you have the vent hole, clear that out. Put the spacer into the seal, then onto the axle, trying to put the seal in while the spacer is on the axle frequently results in a torn seal (and leaks), because it didn't seat properly.
6. Input shaft seal, this is behind the throwout bearing. Change this if it's leaking. Might be a good idea to check if the clutch arm and the throwout bearing arm is in good shape, these can crack.
That's it, now your transmission should be pretty much leak free. If I've missed anything, do let me know so I can add it in.