are the tyres the problem?

[minerva]

We drove our 98 Forester up the coast for 45 mins before we could smell a burning smell. We noticed the car had some drag or it seemed the brake was on, even while going down hill. We pulled over and the car shuddered to a stop. We let it cool for a bit, and pushed on for another 10 minutes before smoke billowed out the back. RACQ said there was nothing they could see and when we started again it was ok.
Drove back from the coast and around the same time it happened again, smell, smoke, braking and the shuddering to a stop. This time something caught fire.
Long story short, put the fire out, put it into the mechanics, had a new diff put in and have attempted to drive it down the coast again and THE SAME THING HAS HAPPENED! (minus the fire this time).
We're at a loss as to what to do. Many mechanics are telling us different things - different sized tyres does something, dodgy diff, brake is on etc.
I've been perusing the forums but can't find anything.
Anyone had any trouble like this? Anyone know how to fix it without replacing everything to diagnose it?
Thanks.

[TOONGA]

this problem can be caused by the wheel bearings being worn and causing the axels to sit on an angle which in turn causes the brakes to be forced against the disc

also you say there was a fire, where was it? thats probably the main point of friction

worth looking into

TOONGA

[steptoe]

you mention different size tyres. Have you got different sizes and or brands fitted? Recent post revealed awd like yours should have same brand as well as same size same wear to avoid stuff with diffs and gearbox

[subarooster]

I would have guessed wheel bearing? only because this has happened to me before. Just check if one of your back wheels has EXCESSIVE negative camber. thats how i knew mine were buggered. :P

[AlpineRaven]

Or wrong diff ratios?
Cheers
AP