I have a 92 v6 5-speed. I do the same as you, have fun with it sometimes and other times drive for mileage. If I'm just cruising down a road with no traffic, stops, turns, etc, then I'll put it into 5th gear at 35mph. That speed is just fast enough to keep it driving smoothly. But when I starting from a stop and shifting, if I want to take my time I will some times shift as low as 1300rpm.
Just like what others have already mentioned...Get a feel for the truck, shift at a low rpm, and don't shift it so low that it stalls out or drives rough

 

i have a 96 dakota an it has the shift light but mines comes on at 2000rpm an from 2000rpm to 2500 going for 4 to 5