I called my local Dodge Dealership in Gilbert Arizona & they said that the Shift Solenoid A on a 99 Durango, 5.9 with 4wd is Dodge part # 4617210. Had it in stock for $113. Autozone did not sell & NAPA had to special order it for $123.
Parts person said it looked accessible by dropping the pan.
Go into your local dealership & ask them to show you breakout picture in their computer. Don't know if that part replacement is shown in any of the after market manuals.
Let us know how this goes & if it was much of a challenge.
Good Luck.