My 99 Expedition just died while driving. After towing it home it now wont even turn over. I can jump the starter and it engages and I have power to dome light and headlights but when I turn key nothing ( but lights dont dim) Im thinking T/R sensor , Ignition switch or Key switch? Confused on how any of these would first cause vehical to just quit while driving, it then would turn over fine but not start and know I have nothing when key turns to start position. Replaced fuel pump--but old one was good 100$ down the tube.

You might make sure that the transmission is in park. Sounds like the shifter interlock might be engaged. The tow company might have taken the trans out of park for the tow?

Sounds like either a bad connection at battery or starter. Or faulty starter itself, you can try to jump the starter feed and solenoid to engage the starter yourself but make sure the car is in park.

jumping across the solenoid will spin the starter but will not engage the pinion in the ring gear. Have a load test done on the battery as I suspect that there is a bad cell and you are not getting a full charge. If the battery is faulty then there is always enough power to turn on dash lights but nothing left to run the high current solenoid and starter.

Getting spark? Power coming from the battery I think you meant to say, if I am correct. If it won't jump start, sounds like a cable from battery to vehicle is not connected. Igniton switch connection or fuse maybe. Check the daisy chaining of the negative battery cable for clean contacts. Also a starter relay could be bad. Just some more suggestions to add. Don't drive to far from home until repaired. Towing cost too much as you know by now.
Never did like remote locks, We as humans seem to be getting more and more depended on convienances that cause us more inconvienances. Could try a positive cable to starter to check if it will turn over the starter. Voltage regulator could cause battery not to charge while driving. When car dies, do lights work or they dim from engine sucking battery dry. Just some suggestions.

Check the cleanliness and tightness of the battery terminals. Loosen, twist and retighten them. Also check the cleanliness and tightness of the black earth cable where it attaches to the engine block/bodywork. A bad earth can cause the symptoms you describe.

A quick check you can do is to connect a jump cable to the battery negative terminal - the earth - and connect the other end of the cable to somewhere suitable on the engine/bodywork.

If the car starts no problem with the jumper cable - it's pointing towards a bad earth/connection somewhere on the battery earth

it sounds like you have a bad ground. follow all your ground wires. should be one to the engine, body, and maybe frame. disconnect them and clean the surfaces with sand paper and reconnect

Most likely your battery is dead, the clicking usually indicates that. The battery may have the required voltage however, it may not have enough amps. Try giving it a jump start.

it sounds like it might be the ignition switch, is the starter ok or the starter relay

Well odds are that your blazer is fixed by now. Either way sounds like a bad battery, that's why you need jumps all the time. Why it would die, there are too many possibilties off hand. So what was wrong with it?

resistor.............tell the mehanic