Dern right, I plan to never bother with OEM style again, only Marlin's.
1. Watch your installation technique, as mentioned. Don't allow the shaft to lever on the seal as it's slid in position.
2. A very remote possibility is also a bent shaft. However, assuming it wasn't leaking prior to the ARB job, I'm going with #1.
I personally use my greasy fingers as a fulcrum while sliding the shaft home. (Jesus, that sounds like a line for rChat) I understand it's a pain to get the splines engaged with the diff, but I push one hand up while pushing the other down (and vice-versa as needed) while also turning in a circle and pushing in. It eventually pops in place. I hear of guys using coat hangers bent into a hook, but that's never seemed stiff enough for the leverage needed.
Edit: After a reread of the above, it seems I didn't describe anything you aren't already doing. The difference might be in really focusing on using your hand nearest the seal as the fulcrum, instead of the seal. Sorry, no real moment from me, it seems.