I have a volvo penta aq 125b/ 270. Prop wouldn't engage, but would turn slowly when shifted from forward - reverse. I've checked the upper and lower shafts like you said and found out that the shaft from the engine has come away from the engine. I've tried to push shaft back toward engine and it aligns. When I reinstall gearbox on lower unit, it pulls back out of the engine. How far should the splines come through slip joint towards the double cardin u joint? If thats what I'm looking at. I have no repair manual or the means to get one. HELP