It does sound like a ground problem. Odd problems like this are almost always grounding issues.
Do you have a test lamp? See if you can connect a test lamp across the brass base of the bulb and the negative post on the battery. Almost certainly you'll need to string a jumper wire from the battery to the rear of the truck, but you only need it long enough to do some testing.
Connect the test lamp between the bulb base and the battery post, and then have a helper operate the turn signals. If you have ground problem the test lamp will light up when connected to the base of at least one of the bulbs.
If it were me I'd first double check your wiring job just to be sure. Provided I still thought it was correct I'd just assume the grounding was bad, and look for a way to fix it. The wires you've replaced are only the hot side, not the ground side, so verify that a good electrical path exists between the light bases and ground, and that the ground straps between the battery and frame, and frame and body are all in place.
