If your talking about a tail gate, get some heat on the torx headed bolts before you try to take them out.It will pop the rust and loctite loose.It'll burn the paint too, but if it's all rusted,I doubt you have much paint to worry about.
My brother replaced the liftgate on the 85 CJ-7 I sold him - I still haven't ordered one for my hardtop. The only tip he gave me was don't use a new gasket because it made for a real tight fit. Better compare opening sizes in the old and new before you try putting in the glass in case the glass one is smaller.
"Think your new truck is better, let's go wheeling and see"