Rear end fluid change

[BDK]

Just picked up my new 2001, Z71 with a locking rear differential.  Does anyone out there know when to change out the fluid in the rear end with just normal driving?  The instruction manual states that it should be changed "after the first 500 miles of trailer towing".  I don't tow a trailer and would really like to start off right.  When would you change the fluid?

[RyanbabZ71]

check this out

 

http://home.att.net/~m.prendergast-allieds...erado_axle.html

 

i changed mine at 18K