The axles should be just fine for a backhoe. The Massey 202 & 203 work bull, 20, and 20C all had the same rear axles and had factory mounted full sized backhoes. The Massey 20C had a 54A backhoe that weighed 3,545 lbs.
The issue is that these backhoes were mounted on a full length subframe that distributed the forces of the backhoe. The 3pt linkages on a 135 do not fair well, they like to rip the pins out of the axle, causing the whole axle housing to be replaced. Do some research on the form on the stress 3pt backhoes put on a tractor. You will also need a loader or serious counterweight in the front to counteract the hoe. I would strongly advise not putting a 3pt hoe on your 135, or any other tractor.
I have a 20C with a Bradco 11MD mounted on a full length subframe, and it does ok. Still not as nice as a full sized backhoe.
