I have a Toro GTS-5 self propelled rear wheel drive mower that's 13 yrs old. The left rear wheel will not fully engage. The right rear wheel engages so the mower pulls to the left. I took off the differential cover and all the gears look fine. I pulled each wheel and the pinion gear and rocker key look fine but when the left wheel is under pressure, it won't engage. Does anyone have any suggestions. I greased the wheels already.