Small Engines (Lawn Mowers, etc.): John Deere STX 38 Lawnmower Transmission Repair, transmission gears, forward speeds

Question
I own a John Deere STX 38 lawnmower, which is probably around 10 years old.  The transmission on the lawnmower has 5 forward speeds as well as reverse.  Recently, the transmission/gears has started "slipping" when mowing my lawn.  Can you confirm the likely issue with my lawnmower?  Is it feasible for a mechanically-inclined person to fix himself with the proper repair manual?  Or is it better to simply take to local dealership?  How expense is this repair (parts = fix myself or parts & labor = fixed by dealership)?  Sorry for all the questions, but thank you so much for the help. Thanks.  

Answer
It's probably the motion belt slipping.  If this is the case, the belt is probably $25-50.  Replacement is a hassle, but not outside the DIY'er realm.  Just pay attention to how it's put on when you remove the old one.  Paying for replacement is probably in the $75-100 range.  Thanks, PK.