If your trans was low, there has to be a reason. I'd strongly consider doing a full check underneath, and confirming there is no leak anywhere. I'd also take a look at the fluid on your dipstick and see what it looks like. If it's red (like the fluid you just added) you may be looking at someone changing it prior to the sale but not getting it full enough. If it's burnt smelling or it looks much darker than the new stuff you added, drain the pan (there's a plug), drop the pan, replace the screen and clean the magnet, replace the pan and refill with trans fluid. Then disconnect the return line coming from the trans cooler to the trans (disconnect the rubber line from the metal return line right at the passenger front of the motor - but be sure you disconnect the return line, not the feed line) and crank the truck to drain the fluid into a bucket. It helps if you mark the bucket first for quarts. As you get a quart out, shut off the motor and add a quart through the dipstick tube. Keep doing this for about 8-9qts. When the fluid coming out changes to bright red from the darker "old" fluid look, you've hit new, clean fluid, and your entire system is changed out, and you're done. Only other thing I'd say is swap the clamps on the trans hoses for worm clamps or fuel injection clamps, and go ahead and replace the rubber hoses while you're in there.
And, to answer the other question, yes, being a quart and change low will cause issues with the Titan. Hope you caught it in time to not do any damage.
