Lopsided GMC Pickup

Could any of you please comment on a problem I have? I own a 2000 GMC Sierra 1/2 ton pickup, 2WD with milage of 68000 kms (42000 miles).
Awhile ago I noticed that the left-rear side of the truck appeared to be
lower that the right. I took measurements and found that the left was 1 1/2 inches lower than the right. I took these measurements from the bottom of the wheel rim to the fender wheelwell. (Measuring from the ground may not be accurate due to uneven tire inflation or uneven ground).
I brought this to the attention of my GM dealer who checked it out and reported that the rear of the vehicle was level. I took measurements again while at the dealer and discovered that the difference was only 1/4 inch. So something had changed while driving from my home to the dealer's site. I drove home and checked again. This time the difference was about 1 inch. What's going on here?
I have a theory. There is almost no weight in the bed of the truck so the back end of the truck is light. What if there is dirt or other foreign matter between the spring leaves? As I drive over uneven road surfaces the springs flex. Due to dirt between the spring leaves and because there's not much weight back there, the springs don't always return to their original position. What do you think? Have you heard of this problem before? Perhaps I'm worrying about nothing but my concern initially was that there was a weakened spring. I've never carried any heavy load in the bed.
I thought I might attempt to spray liberal amounts of WD40 between the spring leaves in case my theory is correct. Any thoughts or similar experiences?
Thanks to all.
Clark Martin
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Clark Martin wrote:

Do you have gas shocks? If they are oem at 42k miles it's time for new ones.
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I'd also say to check the rear shocks first like the other poster mentioned.
Did you buy the vehicle new or used?
Possibly there is some flex in the bed / or frame area. Check for loose bed mounting bolts. I saw a new Chevy truck awhile ago & they didn't even have wheelwell liners in the rear.
========Harryface ======== 1991 Pontiac Bonneville LE 3800 V6 ( C ), Black/Slate Grey _~_~_~_~276,925 miles_~_~_ ~_~_
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