Liquid nails will woratleast it did on a truck I sold last year. Whatever epoxy you choose, you will want to let it set-up before driving.

You could also wedge a piece of wood or thick rubber behind it, using a set screw through the rib to hold it in place.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed