Stalling/Loping in Gear

I've had the motor stall out on me a few times when I put it in reverse. It's not a constant thing and it's happened both hot and cold. The RPM's drop slightly when it's put into gear. As in it's noticeable without a tachometer. I've changed the IAC valve, cleaned the throttle body (w/o disassembling it), cleaned the electrical connectors on the throttle body, and changed the transmission filter (heard they have a habit of dropping and causing this). Once out on the highway, the truck drives great for the most part. Though it does lack power when going up steep inclines, but hey, it has 237k miles on it. Looking for other suggestions to nail this problem down. I'm in the process of getting the money together for a motor, but being it runs fine on the road, I'm guessing there's another issue when at idle and putting it in gear.

Almost forgot, both in-tank fuel pumps have been replaced, as well as the fuel filter.