I am looking to replace 2 rims on my 88 Jeep cherokee.  I am wondering if anyone new the correct backspacing for that year.  The rim model number on the vehicle now is 15x7 JK20  

the stock is 5.5" if your going to a 11.5-12.5 wide tire 4.5" is what will work or rubicon wheels also though they are 5".  

Well the tires on there are P235/75R15 if that helps  

and Have also had someone else, a mechanic from a jeep dealership tell me that the backspacing was 3.750 inches on the '88 Cherokee  

nope, 3.75 will stick the tires outside the fender flares.

for the 235's the stock 5.5 will work just fine, they may slightly rub the front lower control arms, won't hurt/dammage nothin...  

if you dont believe him then just take the tire off, run a straight edge across the rim on the brake side of the rim and measure to the wheel mounting surface.. but im sure hes right if the're stock rims  

With less back spacing, the tire will move closer to the lip of the fender when turning.

For example, the same size tire on the same width rim will rub when turning with 3.5" of back spacing but will not rub with 5" of back spacing. This is called "scrub radius".

Cherokees have a very narrow wheel well opening and scrub radius changes have a huge affect on Cherokees.

The stock wheels are 15x7 and have 5.5" of back spacing.