No fuel flow from tanks 7.3 diesel

The black reservoir/fuel tank selector unit you are referring to is the fuel tank switching valve, which is designed to switch between the two fuel tanks in your 1988 Ford Ranger XLT 4X4. This valve is responsible for directing fuel from the selected tank to the fuel pump and engine.
The second fuel tank selector unit you mentioned (part number 5sv2k) is likely a replacement part for the original fuel tank selector unit. It is not intended to replace the fuel tank switching valve.
The dual fuel tank system in your Ford Ranger requires both the fuel tank switching valve and the fuel tank selector unit to function properly. The fuel tank selector unit allows the driver to switch between the two fuel tanks, while the fuel tank switching valve directs fuel from the selected tank to the fuel pump and engine.
If the black fuel tank switching valve is leaking or malfunctioning, it should be replaced to ensure proper operation of your dual fuel tank system.

indicates blockage in the line back to the tank
could be a venting problem for the tank not allowing full fuel flow as indicated in your statement
check in the tank or filter for a black/grey sludge
That is a moss that grows from water in the tank and blocks lines , filter and eventually injectors. use a diesel fuel conditioner to remove it and always keep the tank full to prevent condensation that causes the problem in the first place

Yes, the switch is usually a solenoid, check to see if it is getting proper voltage and if so, listen for it to click, the clicking is it engaging.

Check the fuel filler cap is correct and venting as it should . Check for blockage in the fuel line pick -up in the tank as it is a fine metal filter that rusts and blocks the fuel flow to the pump.. I take it that by you statement of diesel fuel solenoid that it is a diesel engine. To that point the fuel tank runs at an elevated temperature and when cooling gets water condensation in the tank. This promotes moss growth which gets through filters and generally reduces performance of pumps and injectors.. However it will block the pick -up filter as I said. If you find this to be the case the moss is treatable with compounds put in the tank. To prevent a re-occurrence simply keep the tank full and never less than 1/2 full. NO air space = no condensation.= no moss.

I am guessing that your truck is a Diesel. There is no electric fuel pump on the diesel models. The fuel filter light comes on when there is not enough fuel pressure. Possible problems include the selector vavle not fully switching, and/or an obstruction in the pick up in the front tank.

the selector valve is located underneath the truck on the inside of the frame rail on the drivers side If I remember right. Try swtiching to the front tank and tapping the selector valve assembly. If that clears up the light then the selector valve is sticking and needs to be repaired or replaced.

JUST DRAIN OUT GASOLINE IN FUEL TANK.CHANGE FUEL FILTER AND.FILL UP TANK WITH DIESEL FUEL. ENGINE WILL RUN ROUGH UNTIL ALL GASOLINE LEAVES THE FUEL LINES.

Sounds to me like you're sucking air...does the FUEL FILTER light come on at all? Could be an "O" ring inside one of the quick connects on the fuel line selector or at the tank (if it does it on both tanks, it's the selector valve). Other than that, track your lines all the way up to the filter...if any look rusty at all, it could be sucking air from there....it doesn't have to leak to have a hole in it. The other issue you may have is the "sock" on the end of the feed line for the sending unit in the tank could be old and brittle or clogged up. Give all that a try....

you might have a leaky fuel line

try loosening the injector lines a little then crank it.it probably has air in the lines and wont blow through the injectors,once fuel comes out while cranking,tighten them,if this is the problem it should start but run rough untill the lines are tight,it shouldnt hurt much to get a little fuel on the engine

time to clean out the system
you should probably try running reg diesel through the lines
so drain the fuel tank and put some reg diesel in rem the fule line recently replaced and allow it to pour our into a bucket then after the fule starts running there then hook line back up and start pumping it through the injector pump remove 1 of the cylinder lines and see if you have fuel there using a clear hose and a bucket
attach hose to end of fuel injector line and see if it is pushing fuel through the injector body
remem ber you are still pushing reg diesel

now that the system is clean and clear hook all lines up and try and start the truck up on reg diesel
if it runs then your off to the races
if there is no fuel check the fuel filter it may just be full of gunk
or the pump might be cooked
but rem line b4 filter and check for flow