1991 Ford Thunderbird AOD transmission is not doing so well

I recently (about 7k miles ago) had my transmission and differential rebuilt due to an incident which completely trashed it, and the company who rebuilt it gave me a 3 year 30,000 mile warranty on it, but their service is absolutely terrible.

The problem I am having right now is that if I accelerate quickly from a stop, but release power while I am still in first gear, the car makes a loud thud as it stops accelerating. They claim that this is an engine related issue, and they want to sell me an overpriced tune up (spark plugs, inspect/replace wireset, clean throttle bore, inspect/replace distributor cap & rotor, inspect/replace air filter, fluid top-off and a bottle of fuel injector treatment) for $180 to fix this but I know for a fact that there is absolutely nothing wrong with the engine, and a minor tune up would do nothing to fix my problem. The problem only occurs in first gear, and only when I quickly release the gas pedal.

Also, the transmission fluid is very far above the full mark(it always has been since the day my transmission was rebuilt) the guys at the service center claim that this is acceptable, but I suspect otherwise.

I am a poor college student. I am very meticulous, and consider myself at least slightly more car-educated than the average person, so I would much rather take the time to try to fix this myself the right way, instead of paying someone to fix a car they don't really care about.