8-3/4 Noise After Rebuild..What's Normal??

It was NOT a good shop that built your rear end because a good shop doesn't just not put friction modifier in with the oil in a sure-grip rear end. Plus, they haven't given you the paper work AND they IGNORED your requests for certain things like the oil. I installed a set of 3.90 gears in my old 7.25" rear end about a year ago and didn't set them up properly and didn't break them in (I was too excited I guess and didn't realize the importance of proper setup and break in at the time) The gears whined LOUD on accel and decel, people thought the car was supercharged lol and were quiet when cruising at a steady speed like you describe. I lost a tooth off the ring gear after about one week. Anytime I had nasty grinding noises and gears trying to lock up while turning, it was because my spider (differential) gears were completely destroyed and broken apart. Forget "performance" shops. They are almost all the same. Their attitude is that they are racers and know everything about racing so specs (like pinion depth and backlash) can be done by feel because they are THAT good with racecars and they don't need to worry about paperwork and customer service because it's not a fleet service shop or dealership, it's a racecar shop and you're just an easy going guy with a racecar. That's total bullshit. Take your car to a dealership or OEM type well-known cahin shop where they will listen to your requests, use a dial indicator to setup your gears, check the wear pattern with grease on the teeth etc. and also provide you with you paperwork and give you a LONG warranty period. Just because it's a performance car doesn't mean it doesn't need to be setup using tight OEM specs etc. On the contrary, it needs to be done even better than OEM would have it. Often, guys in race shops have been building cars and racing for a long time, but have no ASE certification or other certifications. Guys at dealerships and well known chain shops will have proper training and do things right. They'll also treat you better. As you can see, "Ricky Racer" type guys piss me off to no end because they don't do things right. I used to be that way, but I only worked on my own car, not other people's cars. Get your money back and take the rear end to a place that'll do it right. My 904 trans was done at a race shop and it blew 3 sprags, needing a new case each time, and now it flares up when shifting from 2nd to 3rd gear under WOT. It's only behind 220 hp in a 3200 lb car....Not a huge challenge. Stick with a dealership or OEM chain shop that'll do it for you and you'll be much happier. If left much longer, I guarantee your rear end will blow up. It'll probably be under full throttle and will shock your transmission badly and blow your sprag clutch.