When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
I went with the 2008 model year diff cover and even did a little custom paint job on it. They are super cheap and have vanes that help keep your diff cool. There are many aftermarket ones but I bought my Ford factory unit for about $29...how's that for cheap?
Custom painted(by me using high-temp rattle can paint and clearcoat) 2008 Ford F-250 rear diff cover:
hopefully you didn't use an actual gasket. there is a specific RTV for differentials. Stuff is thicker than normal and is resistant to gear oil breakdown
I have the Mag-Hytec (front, rear and trans pan). All three use an o-ring style gasket. I have not had any leak problems and they have been on for over a year. The best part of the diff covers is that you have the ability to check the level and add/drain without having to remove the cover. Thus eliminating the worry of the gasket leaking.
Pull an old bolt out, go to hardware store and get new bolts 5/8-3/4" longer. I went with the socket head bolts and stainless washers. I didn't think of it at the time but I should have gotten stainless socket head bolts as well.
I went with the aluminum cover from Ford as well. Instead of RTV I used a gasket from LubeLocker,($21), as it's reusable. I've used them on all of my vehicles, never had one leak.
My 2 local big box stores didn't even have the right size bolts. I went with these Armor coated bolts( Ultra Coated Alloy Stl Sckt Head Cap Screw, 5/16"-18 Thread, 1-1/4" Length)and washers(Ultra-coated Grade 8 Steel Sae Flat Washer, 5/16" Screw Size, 11/16" Od, .05"-.08" Thick) from McMaster Carr.. McMaster-Carr McMaster-Carr