99 Envoy (4.3 automatic 4x4) needs a seal replaced in the transmission extension housing at the rear drive shaft. I've got both the Haynes and Chiltons manuals. One says to apply non-hardening sealing compound to the seal before inserting it into the housing. The other manual doesn't mention this. Should apply the sealant? What kind of sealant should I use? Just a tube of Permatex sealant or gasket-in-a-tube from the parts store? Anyone have any suggestions? Also, do I need a tool to install it or can I just use a large socket to tap it in?

just use non hardening RTV to hold it in place while you're reattaching it. It will be fine. Use the proper gasket, not gasket in a tube.

Thanks for the tip. I won't need a gasket. I'm not moving the trans or the rear housing, just the ring seal for the drive shaft coming out of it.