Truck started stalling/slipping in D. Had only gone a couple miles, so I turned around and came back home using 1 and 2. It R'd back into the driveway fine, too.

Some cursory reading tells me it could be anything from a loose wiring harness connection to infinity and beyond.

I'll give business to a qualified forum member first, but am also entertaining experienced recommendations for a fair, honest, qualified technician.