96 Cherokee AW4 not auto shifting to 1st

96 Cherokee Country 4X4 with AW4 trans does not downshift to 1st at a stop in D or 3 but does shift to 1st manually if the shifter is moved to 1-2 position.  I've adjusted the cable by retracting it and manually opening the throttle, I drained the trans and refilled, and I added Lucas trans fix awhile back.  At first the lack of shift to 1st was intermittent - usually after the trans was warm, but now it is a constant. Could it be a bad TPS or some external electrical function, or is it most likely internal?  I have a new filter and plan to clean the pan and install it, but need to know if there are internal things I can do while the pan is off.  BTW, I went looking for the trans control unit under the driver's lower panel according to the 1996 OEM repair manual and couldn't find what the manual described.  Where is that puppy on a 96?