I had a 800 Bolens which would be the previous models to the 50 series. I am guessing that the differential lock on the 750 is on the right side of the tractor on the end of the axel and you turn it in until it gets tight? I never had any problems with mine but just guessing it would have to be a clutch pack in the rear end or possibly a broken axel shaft, but as heavy built as those are that would take some doing to break an axel shaft. I have original shop manuals for the 600/800/900 and 650/750/1050/1250 series tractors. I'm not sure where I put it but I've got it stored somewhere. I will look into it to see how that actually works. The most I ever had to do is straighten a shifting fork that got bent by someone forcing it into a gear, they have very large gears for a tractor of that size.
Those were very heavy duty tractors that simply didn't wear out easily.