Most likely a drive motor, look at the top of the page on my 751 post. 751 and 751s are pretty much identical, same part number for drive motors i believe. i had both of my drive motors rebuilt and it solved the problem. I could be a sticking shuttle valve. If you want to test them for sure remove the plate on the right side of the machine with 4 bolts. its between the wheels. you will probably have to remove the rear wheel as well. You can undo the hydraulic lines (mine were 45 degree) and very hard to get to on the right. It takes a 1 3/8" wrench to fit the large lines. you can buy a set of caps for about $15 to cap the lines off. Then turn the machine on and pull/push the right stick. it should want to stall the motor. It will feel/sound identical to your left stick with the traction lock on. If when you pull the stick back it doesnt bog the machine down the problem lies in your hydrostatic pump. I would say 99% its your drive motor from my experience. You can try to replace the case drain filter but i did that and it didnt help. usually it will overfill your chaincase if that's the problem.