Tuning transmission

I have been using a predator for a while, and have multiple upgrades done to my truck:

2001 Chevy Silverado 1500
5.3 V8

Pacesetter long tubes and off road Y (no cats or rear o2's)
Dyno max muffler
MSD 8.8 wires
NGK plugs
Powerbond 25% balancer
K&N filter (no cold air intake crap)
and of course the predator


The rest of the truck is stock. What i'm running into is, the transmission constantly kicks in and out of passing gear when i'm hauling my boat. First off, yes, i'm pulling in tow/haul mode. The boat weights about 1800-2000 pounds, so it's not a heavy rig. Here is the trick though, if I pull the boat just going 50-55 MPH, it holds at about 2k RPM and it runs strong and doesn't kick down a gear no matter how big the hill is. If I go any faster than that, it will go to the next higher gear, and when there is any little change in road elevation, it drops a gear. It's quite annoying, and I know it's going to wear out my tranny. I also bumped up the speed so when it's running in 4th, I was holding the same RPMs, around 2K. I figured the power band was right around that RPM. Well, that wasn't it. It kicks into passing gear continuously even then.

So, I know I need to adjust the transmission with the tool to fix this, but I don't know what I need to change to fix the problem, nor do I know which way to go when I make the adjustment. Please help guys!