burning rubber smell

When climbing hills, mostly on the hiway, I smell burning rubber. Any ideas. 91 S-15 Jimmy 2dr 4x4. The tires are not rubbing, but I do have a toen left front outer axle boot.

is the belt slipping? sometimes when an alternator goes bad they get very, very hot. also check for anything touching the exhaust. i don't think it's the boot. a burning clutch or brakes smell kind of like rubber.