ABS light on after replacing the front and rear brake pads .

have air in abs block. check for bleader valve on block,

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.

just because the rotors are new dosen't mean they come out of the box 100%,you might have one that is running out,causing the pulse,most pften it is recommended to have them turned on a brake lathe prior installation (saves you the trouble of doing the job twice)

Try re-bleeding ALL wheels starting with furthest away - pass rear, then driv rear, then pass front and driv front. That should get your pedal back. To reset light you need ABS reset tool and it's EXPENSIVE. Have a good competent shop reset it for you. Body shops can reset them also.

The grinding sound when braking is almost surely worn brake pads. If an abs sensor is bad it will set the abs light on. Heres an easy way to figure whether to look at the front or rear brakes first. When braking, take notice as to whether the vibration is felt more in the steering wheel or the seat. If it is more in the steering wheel take a look at your front brakes first, and obviously if it is felt in the seat look at the back brakes first. Both front and rear need to be checked out because the pulsating you are feeling is due to rotors or drums that have been heated up and warped. In order to fix both problems, you most likely will need brake pads and rotors/drums. If you just replace the pads, not only will the pads wear away faster due to the damage to the rotors/drums, but your vibration issue will not go away and infact may intensify.

Try having the dealer flush and bleed your system. It is very hard to bleed ABS systems yourself and have safe brakes that still work afterward. Bleeding non-ABS brakes yourself is easy not the same for ABS brakes. Valving, sensors and what-not require a tech and the correct equipment in my opinion. You do it wrong and you could ruin your ABS system. Do that and see if they firm up. I replaced my brake shoes/pads at the same time all new everything in back, drums/springs everything and new rotors up front. then I had the chevy dealer flush, refill and bleed system. Stiffer pedal and brakes work better. Keep in mind the brakes on 99 Tahoes are inaedequate, require new rotors often, heat up and fade/glaze pads regularly. I replace my pads long before they wear down because they glaze up and start fading early. I'll rough em up once maybe, next time, new ones. Every two brake jobs, new rotors for me. Just how it is. They will stiffen a bit and work better but they will never be awesome brakes. Just how it is on 99 and earlier Tahoes. Hope it helps. Very important to bleed correctly though. I'll bleed my 83 Toyota 4x4 myself but not the Tahoe.

Check to see if your brake pads and rotors are in good condition. Or if you have replaced the pads yourself and have not reset the wheel sensors this would cause the ABS Antilock Brake System light and the ASC Automatic Stability Control light to stay on as the ASC uses the ABS brakes with computer assistance to control the car

make sure that the abs connect for that side is connect properly and the right amount of brake fluid is there

sounds like the abs needs servicing