If you want to check and see if it is your torque converter lock up, there's an easy test. When you get it up to speed and it begins shuddering, without letting off on the gas, use your left foot to LIGHTLY step on the brake JUST enough to get the brake lights to come on. As soon as that happens, it will drop out of lock up. You should see a 200-300 RPM increase immediately. If the shudder ALSO goes away, well, there's your problem. If it doesn't, then it's not lock up. I agree... there's two main things here. It's either a skip when you are under load, in O/D and towing so you really feel it, or you have a tranny issue. You could stick a scanner on it and see if you have any codes stored which are telling, but just not enough to illuminate the Check Engine Light (CEL). If you don't have an OBD II scanner (they are like $20 on Amazon and work with your smartphone), most parts stores will scan your car for free to pull codes. One last thing I would do... check the transmission fluid level, sure. But also, take a nice sniff of it on the dipstick. If it smells like oil, cool. If it smells like burnt coffee or burnt microwave popcorn... make friends with a transmission rebuilder FAST and find out their adult beverage of choice. Let us know what you find.