Has anyone here successfully replaced the essex sump gasket without removing the engine?

It is possible but a bit fiddly, there is not much room at the back.  You wont be able to remove the whole pan so it is best done in stages.  loosen the mounts and jack the motor up or suspend it from an engine crane to get precious centimetres.  hard part will be cleaning the old gasket off so be prepared for a bit of graft.  I would suggest sticking down the side with silastic and waiting over night,  then the front and once its dry do the back.  Just be sure it's your sump and not the rear main 

Thanks JR. The fly wheel and ring gear appears to be dry/dusty. Is this a good indicator that the rear engine seal is still in good condition?

Sounds good.  I am actually about to do this as I attempt to find out why I'm not getting oil at the top of my motor (I think I installed the main bearing shells upside down

easiest way to do it is to use a engine crane to support the motor and remove the crossmember, Im about to do the same...JR it dosn't sound like like its a bearing problem, by the top what do you mean?? the Valve gear is supplied oil through the push rods which gets it from the tappets. hope this helps O

My understanding is the oil gets to the top from a gallery that goes through to #2 main bearing hence why I think the shell is upside down

YES, it can be done.  I got the whole sump off heres how you do it. 
Drain sump and remove dipstick, loosen all sump nuts.
Remove top rad hose and disconnect fuel line at the x-member
Remove swaybar mounting brackets and put a jack under the x-member, loosen 6 bolts
Put another jack under the gearbox bellhousing then remove  the 2 nuts inside the engine mount triangular holes
Jack the gearbox up which will raise the motor, remove the 6 x member bolts, lower the jack under the xmember, voila clear space
remove the sump pan bolts, then remove the oil pick up.
Sump will now pull out, easy peasy 

Aah, good stuff, very helpful JR.