360plumber

Looking for some help. Bought a 2005 F-150 XLT 5.4 4x4 and the rear end chirp/squeaked and slightly bound up when I test drove it (about 4 months ago) but I took a risk and bought it anyway. Right after I bought it, I promptly serviced the rear end with Redline synthetic and (2) bottles of Ford friction modifier. Gear oil I pulled out was pretty clean. Anyways, long story short it binds up and chirps and squeaks every time I start and stop. I had my mechanic look at the u-joints and they claimed they were good. He suggested maybe I lube up the slip joint? I still have figured that out. Back to the problem, I can be stopped and "get on the gas" and it will bind up, chirp/squeak and then get off and rolling. I can also real lightly get on the gas and kind of put my foot into the throttle several times and it will c/s once at 5mph, at 10 mph and at 20ish mph. The noise is definitely coming from the rear end, however I can't tell if it is really louder from the left or the right. A part of me thinks it could also be the brakes? I don't know, but I would appreciate some comments if anyone else has dealt with a similar problem. I've searched a little and haven't seen that anyone else has had quite the same issues. Thanks much.