I have a ford explorer with codes B2900, C1145, C1155. I had the ABS ECU replaced ($800). I am still getting the C1145, C1155, C2770, and C1236 codes. What do you think is the problem? Should I just replace the sensors or the whole sensor system (wires and all).=

Before doing that, try removing (Torx E8) and cleaning each sensor with WD-40 and a rag. Also, verify that the Variable Speed Sensor is fine--if not, replace. I've seen a bad VSS cause a host of ABS codes be thrown on Ford pickups.

Can I get this type of stuff done at any reputable shop or do I have to get it done by the dealership?

I'd take it to the dealership.

Where are the front and rear abs sensors. I tried to locate them so I could clean them but could not find them. Do I have to take off the roaders to access them??

Look for the notched "trigger wheel." Then, you will see the ABS wire and Torx E8 mounting bolt.