You first want to check the drive gears on the drive wheels. See if they are worn out. See if the gears on the transmission shafts are worn.
If these two things are ok, then you most likely have a bad transmission. There ar a couple of opinions on whether you should repair or replace these transmission. I usually favor replacing the entinre transmission. Mainly because somethimes the reason the gears on the inside of the transmission go bad is because of misallignment of the bearings and shafts inside. If this is the case you will be doing this again real soon.
This is not a very cheap repair. I could cost you up to $200 or so to get it done. It also is not an easy job to do yourself.
In several cases on units a little older than yours, we just take the self-proppelled system off and use it as a push mower. It just depends on how much you want to spend on repairing the mower.
I hope this helps.
Please click "ACCEPT" or I won't get paid.You may also consider sending a bonus if I have been helpful,Please leave positive feedback this is how I'm rated. Thanks