1991 Ford F 150 Custom 4.9L I6 2WD Automatic?

A junk yard or a spring shop.

The 1991 Ford F150 with the 4.9L I6 is actually a 300 cubic inch engine. THe next size up that Ford made for that year was a 5.0L which is best known for being the powerplant for the Mustang GT also known as the Ford 302 engine. The I6 is an extremely strong low torque monster great for everyday use and trailering. The rear shackle to the leaf spring is going to be cheapest at your local salvage yard. Best bet is to call around and ask questions. Good Luck.

if it is a "I6" that is a in-line 6 cylinder and is 300cubic inches

the other numbers you stated are v8 type engines.

as far as a shackle, look at a local salvage yard or from a ford dealership but that will cost more!

Go to the local junk yard they sell parts for about any vehicle.

You can get new shackles at your local parts store.

what are you asking? 4.9L = 4900cc = 300ci,

490cc = 0.49L=30 ci

where L=liter, cc=cubic centimeter, ci = cubic inch

and 1 cubic inch= 16.387 cubic centimeters

Don't know, but I sent you a link to look at. G'luck.