I run mine all the time without the rear driveshaft.
Get one of those 2 inch diameter rubber plumbing caps with the hose clamp. I clean the tailpiece cover with some alcohol and use a very small ammoumt of grey silicon.
Do not overtighten the clamp as the cap will pop off.
Give the sealant time to dry. You''l be good to go.
I have driveline problem and I have run like this several times for quite some time...

dave h.
'89 Raider V6 5spd;Aisin Hubs;; Gen2 LS: frt. brakes, rear coils;U.C.arms;R.trailing arms;idler arm; rear LSD axle w.disc brakes ;2 battery system for Dog's fan; relocated ECU; custom bumpers;J.Baker receiver;Conferr roofrack; t-bar crank.