99 Explorer transmission - no holdback

I was pulling a loaded trailer down a winding gravel mountain road and put the auto transmission selector in 1st and it was like putting it in neutral. No holdback through the engine at all. If I gave it some gas and got the RPM up to about 1500-2000, I could feel it pick up in 1st gear. Let off the gas and it would start coasting again. Same thing in 4WD low - engine at idle and seemed it was in neutral but give it some gas and it would pick up and give me some holdback.

Been searching this site and a couple others but can't find any reference to this type of problem.

Anybody here know whether its a transmission, PCM, or other problem?

thanks