I love my XJ6, but sometimes think it is a little bit hard on bumps in the road. More than it should be. On flat roads, it's great.

What's the route to take for overhauling the suspension to get it back to its best?

 

Although these cars are often hard on suspension parts I'd begin with a careful inspection of the suspension, bushings in particular, before assuming you need an actual suspension 'overhaul'....which is quite an undertaking.

You might just need shocks, or shock bushings

Cheers
DD

 

im in the process of making diy for the whole front suspension

new stabilizer links and bushing

all new wishbone (control arm) bushing (4x top and 2x lower , 2x lower )

new shocks

new lower shock bushing

new spring pan bolt



im reviving my 96 vanden plas should be the same for you

new stabilizer link top bolts

new bottom shock bolts

new top and bottom ball joints

new rack n pinion

new outer tie rod ends

new wheel bearings

 

If you have any clunks or knocks, then you're probably looking at suspension bushes. As you say that it just feels stiff or solid, then the shock absorbers/dampers may be shot. If they lose compression, they become ineffective and so the car just bounces on the coil springs. Alternatively, if they seize, then the ride will stiffen considerably.

You can jack up and support the car and place a trolley jack under the suspension carrier, jack it up and see how quickly or slowly the shock returns when you lower the jack. It should be a smooth consistent action, in keeping with the jack lowering.

If you do replace the shocks, remember that there are various options, with some leaning more towards a softer ride than others.

Plenty of info on here about rear suspension!!

 

As you can see lots of different approaches one could take. It would be more helpful if you could describe the symptoms that you feel and/or hear.

Do you hear thumping/banging noises when going over bumps?
Does it happen while driving straight or only on curves?
Do you feel any vibration or buzzing though your steering wheel or the car seat?

Otherwise, as Doug suggested, a visual inspection should be carried out first. The rubber used in most of the suspension bushes/mounts will perish over time but is easy to spot if you get the car up in the air and use a pry bar to move various parts of the suspension.

If you are not hearing/feeling any knocks when going over bumps, my first thoughts would be to check the shocks.