How can I fix my lawn mower?

I have a Craftsman Rotary Lawn Mower, 6.5 HP, Model # 917.377061
I seem to have lost power to one of my drive wheels.
I've opened up the wheel assembly and have replaced a small broken detente/spring.
I am pretty mechanical and am fairly confident that I installed the replacements correctly.
I've also checked the drive belt which seems O.K.
I've been thinking the problem may be in the gearbox, but it appears to maybe be a sealed unit (unscrewing all the gearbox cover screws didn't enable me to open it while still in the mower, [gasket cement?]).
From the diagrams in the manual it is hard to guess what might be the problem.
Any ideas?

Simon
July 2010