Driveline vibration

I'm having a vibration issue with my 1998 4wd SC. when carrying loads in the back, I get a resonant vibration in the drivetrain somewhere from the rear. It goes away under deceleration but comes on again when you load up under throttle. Its quite bad esp when combining towing with heavy gear in the van. With nothing in the van or not towing it seems fine...

I have unfortunately been running the van for a while with one worn RH rear tyre and one new LH rear tyre.. Could this have caused diff bearing issues? Or is it more likely to be a worn pinion bearing or universal? It seems only with a heavy load ie the van is sitting slightly lower at the back that it vibrates. Just wondering whether to strip the whole rear end to check it... Or something else I can check first..

4WD all tyres should be the same, try that first.