Ask Smithy Xtra Shudders when towing

My 2004 NP Mitsubishi Pajero diesel auto has done 147,000 km and now shudders when towing a boat under load. I first realized there was a problem when towing my ski boat in hilly terrain, but have since noticed it going up the on-ramp to the ring road each morning. Replacing the engine and transmission mounts made it less obvious, but didn?t fix it. What should I do next?