I had to run out to my truck to check. Under TR I have an "E". So I guess I have the 4R100?
Yes and that is very rare indeed. Almost the only way to find a 4R100 is in a low emissions version of the F150. You usually only see them in California, though perhaps they are sold in the NorthEast also.
As for what can be done to make them stronger? A lot. Do you need to? Probably not. Invest in a good transmission cooler and that is probably sufficient. Changing out clutches, adding additional plates, and changing the sprag just isn't worth it unless you plan on racing or doing some serious pulling. The 4R100 is just plain strong enough in stock form for most uses.
You can also change out the valve body to make it shift more firmly which will improve clutch life.
If you want to get a bullet proof transmission, then speak to Gregg Evans and get one of his Monster Box 4R100's. There just isn't a stronger transmission available. They run about $2100 or so plus a $1200 refundable core charge (last I checked). As you do not have a Lightning transmission there would be an additional $500 charge for the upgraded parts.
As I have said though, the 4R100 is plenty strong even in the basic configuration. Add a valve body and a transmission cooler and you should get very long life out of your clutches.