1991 E350 5.8L superwagon needs a rear tank fuel pump replacement. Any suggestions on if the tank needs to be dropped or if it can be done in place. Any other suggestions of other parts while I'm in there... Gauge on the dash always shows empty and stalls while driving on rear tank... thanks

 

I suggest you further troubleshoot for any possible wiring issues first and measuring the fuel pressure with a gauge hung on the fuel rail. Spec is 30-45 psi. After confirmation , yes the tank needs to be dropped. I don't think you will have enough clearance to pull out the assembly with just the tank being lowered. I wouldn't want to be underneath while hammering a drift to release the locking ring.
Don't skimp on the pump or you likely will be doing the job again. OEM is best as an assembly if you also need a fuel sender. Delphi or Bosch are my only other pumps I would install. I would clean the tank's interior and replace the fuel filter.Unless the fuel sender is stuck or there's a float problem , an open circuit with the fuel sender or wiring usually sends to fuel gauge to its mechanical limit towards full. If the gauge works with the other tank, you can atleast rule out the gauge itself.