I have a 2005 Ford F350 4X4 and ABS light is on

The ABS light comes on when there is a malfuntion in the ABS system, not when the brakes are low. On a Ford you more than likely have a dirty sensor or pick up ring on the back side of the rotor. And no you do not have to change the rear drum brakes when changing your front pads but they should be inspected.
Usually you will change 2-3 sets of front pads before the rears need to be changed.

vehicle speed sensor located on rear output case of transfer case

Clean senscer with sand paper, mounted on rear end houseing.

IF YOU HAVE FOUR WHEEL ANTI-LOCK BRAKES YOU MITE HAVE I FRONT WHEEL SPEED SENSER GONE BAD OR A POSSIABLE WHEEL BEARING GONE BAD (BECAUSE ON SOME FORD TRUCKS THE WHEEL SPEED SENSER READS OFF THE WHEEL BEARING.

It will be on the drive shaft and the front of the differential in the rear of the car, the ring will look like a sprocket off a bike and the sensor will have two wires going to it.

it is located on each front wheel and the rear axle shaft for thee rear you may have broken the sensor or moved it where it can't detect the wheel turning.

you will have to get the abs computer tested for codes, unfortunately these days things are almost too much for us backyard mechanics!!!

All the hubs on the car have the abs sensor built into the hub. The abs system has to be re set to read the sensor . The rear hub bearing if noisy should be replaced. There are wiring harnesses that connect to the abs sensor. The harnesses to each of the abs sensor should be replaced.

Id get a mechanic to look at it. Sounds like a rear end bearing problem. Axle bearings pumpkin bearings.

I have heard of the 8.8 rear ends premeturely wearing out. Did you buy this car new or used? If you bought it used, you never know how the previous owner drove the car. Remove the rear cover and inspect the oil and gears inside. If the oil smells burnt, the rear end is most likely junk.