How do you replace the Belt Tensioner on a 1998 Ford Ranger 4 Cyl.?

There's no point in buying the pulley if it's too tight. Your mechanic should not 'think' it's too tight, he should have verified whether or not it is, especially if he just checked it. If it truly is too tight, you may be able to free it up. Sometimes you just have to spray a bit of WD-40 or similar fluid onto the swivel and work it gently. CAUTION: sometimes a seized tensioner will come apart when you work it, depending on how corroded it is internally. Also note that htis thing has a spring inside of it and that once it is free moving, it should apply tension automatically to the serpentine belt.