I had the shift cover off where the shift shaft is. Has some grease in it. Didn't look rusty. If I leave the cover off and put it in gear would I see anything in there to let me know if it is jumping into gear

 

Possible that you could see if it is. If so then could be a problem on those blamed wave springs that go on either side of the wide shift forks inside the gear case.They may be collapsed over time not allowing the dog gear to fully mesh with the forward gear until you give it throttle. This would involve tearing into the gear case and checking things out. Again the 600,700 Sportsman used this gear case along with the Ranger and had similar problems on the Ranger engaging.