Did you grease the splines well last time you had them off/or did the last guy? I once bought a motor that apparently, the last guy hadn't...we tried everything, and I mean everything.
http://www.screamandfly.com/showthre.....help-please.
Finally had to cut a section of the lower unit out... heat and use impact hammer for a LONG time...I pray this is not the case for you and it's as Bigshrimpin says...