Turboman
Mab An Gov
The V8 top speed is 105 mph claimed, and the TD5 is 97mph. It's fine to reduce the speed rating down as far as the manufacturers claimed top speed, so yes I suppose 105mph could be argued but the TD5 has separate figures published so can't imagine it'd matter... That being said 95mph is still not quite enough!
The 300tdi however had a top speed of 90mph so perhaps I would be better to swap these Sahara's over to the 300tdi.
In an attempt to keep on topic though, is there anything I can do, be it on the TD5 or the 300tdi to try and keep the car at least a little bit more stable? pump up the tyres a bit higher best option?
As I suggested before, try about 40 psi in the rears as a temporary measure, and take it a bit easy, Insas will be fine up to 60-70 mph.
Swopping the tyres around with the other disco will be time consuming and expensive, as the wheels are not interchangeable.
It is hard to advise in these cases, as I do not know your personal circumstances, but all things being equal I would agree with the others^^^^^^^^^^^^, best thing would probably be to get a pair of good AT tyres for the rears of the TD5, and get another pair for the fronts when they are worn out, then you will have a reasonably matched set!