89 Dakota. 2WD 3.9 V6. All of a sudden while exiting Home Depot with a bed full of mulch, the forward Drive gear felt like it went into neutral. No forward gear in D, 2 or 1. It does work in reverse no problem. Towed home. Can a bad tranny speed sensor cause this? Are these trannies A500's? No codes stored. Fluid level good, color good and smells like it should.

Should be an A500.
Sounds to me like something broke inside the trans.
A speed sensor would mess up the speedometer, and may cause strange stuff with the motor running, but I doubt it would cause a problem with the trans function other then maybe loosing OD. The system is OBD I, its pretty simple.
I think I would drop the pan and see if maybe the band lever fell out of place, or see if there are piles of stuff in the bottom of the pan. Gene

1st gear in drive is rear clutch and over running clutch in use. If it still doesn't move when shifted manually into 1st you can rule out the over running clutch as the problem. Reverse uses the front clutch and rear band so we know they work. You either have a rear clutch failure or a gear train failure. It's not going to be anything electrical.