Same problem a few years back on mine. I use mine for logging. I fixed it at very low cost and no more problems. Buy 2 new couplers, cut 1/3 off of one of them and weld it onto the other one. You need the coupler to be longer to catch the good splines further back. You will have to have some play in there so you can get it back on. Take some measurements. I also put a section of hose over the drive shaft so I could secure the coupler in place. The problem is the shaft moves around in the couplers and when the shaft and couplers move all the way back you have very few splines catching. So, make the coupler longer and stop the shaft from moving around.