My 1997 ford expedition has no power when engaged to drive, It drives very slow and backfires also

The censor is not seeing it is engaged or it isn't engaging

The light is to come on when the solenoid has engaged the transfer case to the front drive axle. You could have old oil built up in the switch itself down below beside the transfer case and cause the switch to be unable to make the connection as well. I'll catch it for this but some spray carb cleaner and a rag helps in that case. A 'sometimes it works and sometimes it doesn't' could also be a sticky solenoid that pushed the engagement pin into the gear to connect the transfer case to the drive-line. Again, carb cleaner can help once you pull the solenoid.

Could be a switch relay or fuse

That happened to my old truck, an 04 dodge, and I said to myself " chancery, old chap, this must mean a new transmission" , I was mistaken. The solenoid pack in the tranny was bad. I had that replaced for a fraction of the cost of a new tranny and never once had another transmission problem.

It sounds like you need transmission fluid and filter change. This is one of the most overlooked maintenance items. It should be changed every 50k (30k if you tow a trailer) The delay in engaging gear is sign that trans fluid is tired or REALLY cold. In cold weather this is a normal symptom on cold morning starts but goes away when warm. I'd get it changed and soon, before it turns into a $$$ repair.

I have a similar problem that I am having with my 97. The light comes on for 4WD but it will not engage. Have you figured out what your problem might be?

yes-could be number of things-1st chec fliuid level-try to get car scanned cause its electrontrans also-car is 12 yrs may need rebild there--secure a manual or ask a pro to drive it

Is it running rough as well? If so sounds like the timing chain may have jumped.

Possible solution..Expedition uses a vacuum solenoid on the firewall that controls vacuum to the 4-wheel drive actuator. Vacuum "unlocks" the 4-wheel drive, so you may have a leak in the line to the actuator so the 4-wheel drive is always on. Seems backwards, I know.

solenoid switch that engages the 4WD more likely will need replacing. fairly cheap to fix replace.