Hello! So my drivers side running board motor went out as well. I have the extended wheelbase Navigator and there is no Dorman part for that, only the complete assemebly (step, brackets and motor) from the dealer for around $1800.
Well I found
www.runningboardmotor.com and picked up a new motor for $350. (Per his website, they work for both the short and long wheelbase version). I had a shop install it for me for about $175 in labor. There were several issues afterwards that had to be figured out, but once install properly, they work great! And much cheaper than what the dealer wanted. Just wanted to share a fairly inexpensive way to get these back up and running if you want to keep them.
Ed, was also very responsive in emails about our questions during the whole process.