To the original poster. You should be fine, won't worry about it. Swap out another 4-5 quarts next year.

For some general info.
The tranny cross brace has always been in the way of droping the tranny pan on the S-Series trucks. Its not a design flaw, or some other conspiracy. The cross brace is designed to hold the tranny t-case in place so it doesn't crack the bellhousing, motor mounts, etc. The tranny weighs roughly 200 pounds. Hanging that off the engine is alot of weight to go unsupported, toss in another 100 pounds for the t-case and you see why it needs to be supported. To keep things the same, the cross brace bolts into the same location. Either behind the 2wdr tailhousing or behind the 4wdr adapter. Both put the cross brace in the same location ont he frame. This requires only one set of holes to be drilled/punched into the boxed frame. Cost savings in the manufacturing process. Some engineer was thinkning when they deisgned this.

Unfortunately, the down side is that the cross brace has to be removed to drop the tranny pan. If you drive 12,000 miles per year, and change it every 30,000 miles, that is once every 2 1/2 years. It takes 15 mintues to drop the cross brace, if you have the right tools. The only time the exhaust cross over pipe has to be removed is when you have to drop the tranny for a rebuild. It gets in the way when trying to drop the converter cover, and you have to take the converter cover off to unbolt the torque converter from the flywheel.

Now, there is no real reason to drop the pan. A tranny flush is done through the top cooler line at the radiator. There is no reason to replace the tranny filter. It will only filter tranny parts, and if it is plugged up, you have much bigger issues to worry about. The only places the contamination can come from is your tranny. The tranny is not subject to the outside elements like oil, or gas is.

As for adding more detergents, don't bother. Tranny fluid is full of detergents, this is what gets people into trouble when they do a fluid flush on a tranny with high miles on the fluid. The new fluid will clean all the contaiminants off the seals, which might cause them to leak, which leads to slippage or a sticking valve. Over time those seals get hard, and contaimination build up helps prevent the slippage/sticking valve. Heat is the number one reason trannies fail. The heat causes the oil to loose its lubrication abilities, and it causes the seals/gaskets to get hard.

The idle tempurature for your trans is about 175 degrees. The higher you go over that the sooner the rebuild. I have seen a chart on the effects of longevity and heat somewhere on the net, might have been posted on this site. 100,000 miles is roughly 8 years @ 12,000 miles per year. That ain't too bad considering what they have to do. If you take care of it, you can double that pretty easily.

Every tranny tech I have talked to has recommended the same thing.
Install a drain plug and swap the 4-5 quarts that drain out once per year. More often if you tow, live in the mountians, or do a ton of stop and go traffic (like police and cabs). None of them recommend an external filter. Sure the first time you have to drop the tranny pan to install the drain plug, and the cross brace is in the way, but once that is done. Its done. The newer trucks are coming with drain plugs, at least the fullsizes. Just my 2 penny's.