I went to install my adjustabel panhard bar that I bought from GMC puals but it won't fit with the frame sitting on the bump stops. To me it seems like the bar is too short. I put it up to the old bar and it is about 2-3" shorter with the adjustable end all the way out. My question is am I doing something wrong or did I get the wrong bar from GMC puals any help will be great thank you.

OK, you must start thinking back to your days of high school geometry. Hopefully this will help: Picture your truck up on jackstands with the frame up and the rear end hanging down. Now look at your panhard bar. Think of everything as a triangle. Now, if you raise up the rear end under the pumpkin, the panhard bar (if it was a shock) will want to compress as the rear goes up. This means it's length will get shorter as it is in its upward motion. The panhard bar is the longest side of the triangle. So if you are staticly lowering the rear end 4" or more, you'll want the panhard to be shorter to compensate for this. This is where the adjustable track bar comes in. If you use your stock panhard, as the rear end comes up closer to the frame rails, at a certain point there is nowhere for the rear end to go but sideways as it travels in its fixed arc.
Bottom line, you do want a shorter panhard bar, but 2-3" sounds kind of drastic. Are you sure this is the case? On mine, IIRC I have about 1-1/2 of threads showing, but mine came from CPP. Call GMC Paul's and advise them what your findings are. Maybe they did send you the wrong one by mistake.

with the frame on the bumpers do you have the rearend centered?? with tires on the measurement from the inside of the tire to the framerail should be the same on each side

I will check the measurement to make sure it is centered and 67Fleet thank you for the geometry refresher but I wasn't having a problem with the actual theory. Just the application so I will double check the spacing to make sure it is centered I will let you guys know the measurments as soon as I am done. Also thank you both for the help.

ok I went and mesured the axle is only off center 1/16 but the adjustable bar is off buy 2" when fully extended compared to the original one.

you measured tire to framrail right?? NOT pinion to framerail

I don't have tires on it but I measured from the inside of the drum to framerail which equaled about 7 3/8 inches

OK, then you definitely have the wrong bar. It shouldn't be that much shorter.