I'm not familiar with this tiller, but either you have a belt that is very loose, or improperly adjusted idler pulley, or maybe a sheared pin in a direct drive. Clutch adjustment?

Type your model number into Google and look at a manual.

While I don't know about your tiller, I assume that it is belt driven. It is reasonable to assume that either the belt has stretched too much or perhaps the tension just needs to be adjusted. It looks like there is a black plastic guard covering the belt. Remove it and see if you see the belt slipping when the tines hit resistance. If the adjustment still has room, just change it, otherwise bring the belt to an auto parts store and get a very slightly smaller one.

Are you sure the transmission engaging all the way?? I had one few years back and I heard big clunk when shifting to and from fwd and reverse. I would check the cable maybe it binding and not engaging all the way. Could be out of adjustment, corroded and the cable not moving much, ect. wait the travel and see if you can get trans lever engaged more.

Post the Sears 917.xxxxxx number (or whatever it is) so people can look up YOUR tiller instead of speculating on which of the various kinds it may be.