I know nothing about your setup, but could it be a linkage problem, that perhaps can be fixed via adjusting the linkage to the rear end or replacing parts such as bushings?

 

it

@dave_r - How do I determine if it is a linkage problem? Someone else (Evarlandfarm) suggested I verify the pulley going into the trans is turning the shaft it's on.

 

Did you verify that the pulley going into the trans is turning the shaft it's on? As pointed out above, is the linkage positively operating the gear selector?

 

@Everlandfarm - I have not. do i need to remove the back cover to be able to see that?

 

Also check that the axle keys that connect the rear wheels to the drive axles are not damaged. If one allows slippage, it will prevent motion in either direction.
The cue would be the axle turning while the wheel stayed motionless. You can remove the 'caps' to see the end of the axle shafts.
tom

 

OP, you should be able to trace the drive belt for the trans from the engine. There's another one for the mower so be aware. Not familiar with your specific machine, but find the belt, then do what you have to to get to the pulley, maybe removing a tire, usually on the right side. While looking at pulley, operate the shift mechanism and observe what is or isn't happening at the trans. These are pretty straight forward. If you decide to disassemble anything, take pics!!

 

@Everlandfarm - got it! thank you! will try and let you know.

 

While you're under there, make sure to look at the drive belt, it may have stretched or have chunks out of it. Also spin all pulleys to see if they'll turn freely. There's a spring that supplies tension to the belt causing the belt to engage and turn the drive pulley, usually associated with the foot petal. As I said, they're pretty straight forward, the pulley on engine turns, the belt transfers that motion to the drive pulley on the trans via idler pulleys, tension is provided by a spring.