If you try to tow with it as-is, you probably won't be happy with the performance.
That is all that will happen, UNLESS the transmission is already on its last legs, which is entirely possible on a 16 year old truck. Then it is likely that the transmission will fail, but it would fail whether you were running a 3.08 or a 4.10 ratio.
You will simply stop driving it long before anything bad happens unless you have absolutely no common sense, and wouldn't know an underpowered towing situation if it came up and bit you on your arse. Since you had the good sense to doubt your decision, I don't expect you're one of those people.