Good suggestions so far!
A few other possibilities:
8. The easiest one to fix... Incorrect tire pressure in any tire on the car - front or rear - particularly if one tire is significantly different than the others. (Note, TR6 original tire pressure info may still be current, but older car tire pressure info was often for bias ply tires and modern radials will likely need different pressure than the original tires, so owner's manuals and repair manual info might be incorrect).
9. Mismatched tires.
10a. Rack & pinion mountings worn out/loose.
10b. Wear within the R&P unit.
11. Steering column flex joints worn/loose.
12. Occasionally, unbalanced drive shafts and/or rear axles show up in the steering (but I think this is unusual for a TR).
Personally, I'd check #3, 4, 10 & 11 on a car that's had out of balance tires (or any of the other issues) on it for a while... Seems to me vibrations you can feel wiil often cause other parts to wear faster.
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