Walk Behind Lawn Mowers

dear husqvarna,I have a sears craftsman(husqvarna)model#917.380451.since the beginning of purchase 2011 or 2012 I have had problems with the gear case.the problem is when you want to pull mower backwards the wheels sometimes stayed engaged.last year the drive cable finally broke so i replaced hoping that this will do the job but no luck.Brought back to sears but they said nothing was wrong.(3 times).I have struggled with this mower for awhile.I have tried all the tricks and even have come to terms that i have cut grass without engaging the unit(isn't this great)and even then when backing up the wheels still locked(intermittent).I know the unit is no longer in warranty.I have taken apart and cannot see any visible signs of damage.there was oil in unit(black as coal).money is tight and looking for any suggestions that can help me solve this problem.the only visible wear i can see is the 2 metal tabs(tension springs)that hold pressure on one of the gears.can this be the problem??? I know the unit is normally not serviceable but is there anything I can look for???I have 3 questions.
1st question if some parts maybe available can you send me a parts diagram for the gear case.
2nd question what lubricant is in the gear case.
3rd if nothing else and a new gear case is necessary what is the part number because I AM SURE OTHERS HAVE HAD THIS PROBLEM AND HOPEFULLY THERE IS A UPDATED GEAR CASE.Thank you very much for your help.