Considering parts replaced, I reckon the OP's mate is thinking it's coming from the front axle. And like you I'm of the opinion its probably a wheel balancing problem, hence I asked the question.
But hey ho either way, I'd be inclined to check wheel balance is right before I set about replacing parts. Mind you the OP might say that was checked first, need a bit more feed back.
Anyway as you may well have worked out by other threads on tyres
I'm into tyres in a big way monitoring wise [having bought a digital tyre tread measuring device] So before I went on my recent "jollies" to Dorset last week, I measured the tread depth on all tyres.
Now don't sigh, but with the fronts [Marangoni Verso's] having four water channels, and the newer rears [Goodyear OptiGrips]having three water channels. I take four measurements on each front tyre, and three measurements on each rear...I can hear you all saying ....get a life...well actually its all about hanging on to the one I've got
Right, the aforementioned data is entered onto a spread sheet, and can be compared with previous readings. So did another measurement when I got back home. Now I was expecting to do circa 1k miles whilst away, but the grandson was grounded with the runs, so did not travel far from a toilet for a couple of days
Anyway I covered 815 miles, and whilst on the motorway I noticed the steering wheel vibration circa 74-80 mpg. Now the fronts where swapped from the rear axle back in March 2010 with 5.5mm of tread on them, and had no vibration problems, whilst they covered three touring motorway trips, from time of swap to the last one back in September 2010. Now prior to this last trip the front tyre with the lowest average tread depth was down to 3.32mm
So maybe they are liable to go out of balance the more they wear down, or I've taken a knock on one of the wheels over the winter.....who knows. Anyway, the new readings show that whilst the wear on the rear tyres was equal on both tyres. On the fronts, the offside front has worn at more than twice the rate of the nearside front. 0.24mm against 0.09mm
Take from the above what you will, as I know some peeps who think detailed analysis is boring, will have switched off by now
Also as I have not had the digital tread tool that long, I cannot give any detailed wear comparisons for earlier measurements. But given that the front tyres will most probably be down to 3.00mm prior to a jaunt down to Cornwall, end of August. And given the cost of rebalancing tyres near to the end of their usable life, I reckon I'll just replace them with a couple of new tyres, and see if it cures the vibration problem.