Welcome to the Forum.
The correct size Injector return line is 3.2 x 1.9mm, buy it by the meter.
The old part# 615-476-03-26 has been replaced by 605-078-05-81.
The fuel line just pushes on and held on by the barbs on the Injector Nipples. You don`t use any hose clamps.
When you remove the hoses, do not use a knife to cut the hose off the Nipples.
Nicking them could cause a fuel leak.
The last Nipple by the Fire Wall is just a short piece of hose with a metal plug. It can be removed from the old hose and used in the new piece of hose.
It is a bit difficult to just knock off the return lines, the are on pretty tight.
Maybe the mechanic used the wrong line?
A plugged Fuel Filter will not cause the lines to come off.
Differential could be the Pinion seal, and a good majority will leak a bit with high mileage. Usually not a problem unless it is really leaking a puddle
The rear Diff cover could be the problem also. If it is, it`s an easy DIY fix.
Put the rear on jack stands securely. Block the front wheels.
Make sure the fill plug will come out first, before draining the Diff.
Drain Diff.
then place a floor jack under the Diff.
Remove the 4 13mm bolts holding the Diff mount to the body.
Lower Diff down a bit.
Remove the 8 15mm bolts, and the cover will come off.
Degrease the cover good and the mating surface of the Diff.
Use RTV as the sealant.
Replace the cover and Diff mount bolts.
Fill the Diff with your favorite flavor of gear oil.
The plastic necks and the over flow nipple to the expansion tank are pretty brittle after many years of heat etc...
Charlie