Ericnova72's right. If it's the 7.5"/7.625" 10-Bolt they're very weak. If that's what you have try to find an 8.5" 10-Bolt, the 7.5" isn't worth wasting $$ trying to upgrade it as either engine will destroy it if you have good traction.
Housing ID.>> http://www.novaresource.org/axle.htm
or here>> http://dyeager535.topcities.com/diffid.html