2000 QXE-Quest transmission made a popping sound when backing up, and will no longer go forward or reverse. Parking paw works fine and fluid is clean and red. 139K miles, but never abused. Maybe a snap ring popped off or something like that. Now only makes a little noise when shifted into forward or reverse.
Check for a seized brake caliper or parking brake. Not rare on a 18 year old vehicule.
Remove the access plate at the bottom of the transmission to expose the flex plate.
Turn the engine over by hand at the crank bolt to see if the flex plate rotates.
If not, the crankshaft broke at the rear main bearing and is no longer attached to the flex plate.
Yours would be the second Quest I’ve heard this happening to.
Tester