1997 Ford Explorer Clutch and Slave cylinder

My daughter has a 1997 Ford Explorer with the 4.0L engine, 4 X 4. Manual transmission. She has trouble with getting it to shift into reverse and the clutch release bearing seems to make a whining noise. She has had it to Ford 2 or 3 times and had the clutch disc and pressure plate, pilot brg and release brg, as well as the clutch slave cyl. Repalced but this does not seem to help.
Some one told her that Ford needed to put shims in someplace to make it work.
Has any one had problems similar to this and if so what did you do about it and has anyone heard of shims for a clutch? I sure never have.
She is a working mother and needs this vechile for work. I appreciate any help that any one can give.