The best way to identify the 8.4 is to look for the extended and somewhat squared-off “cover” on the back of the housing. The way the “cover” bumps out makes room for larger bearing cap trusses. The third member is attached to the axlehousing with 10mm studs that have nuts that take a 14mm wrench. The older Toyota 8-inch axle and the undesirable 7.5-inch axle both have a rounder, smoother dome-like “cover” on the rear portion of the center of the axlehousing. If you are still unsure what the five-lug Tacoma you found has for a rear axle, pull an axleshaft and count the splines. If it’s 30-spilne, it’s the one you want.