1995 Toyota 4Runner engine just swapped

Hello, my 95 4runner just had it's engine swapped w a used engine. Since the swap, I've noticed several problems that need attention and am looking for advice b4 I bring it back to my mechanic, who just seems he wants to wash his hands of everything and says its running fine. When I got it back, it has a pretty good tranny leak from the front differential, which he felt a bottle of tran stop leak would fix, but hasn't. I'd like to avoid replacing the tranny if possible but perhaps as you hear more symptoms u may think this is related. When the car is first run, after warming up in driveway, my brake light will go on and off, but stays off after a few miles. On the days where it goes off and on, and sometimes when it doesnt, the car has no pickup whatsoever. U can get that engine up to 4500rpm w/o much actual acceleration at all. Not good on the highway going up a steep incline. Also, when traveling between about 1700-2200rpm the engine will buck like crazy unless I lay off the gas and pick it back up again. And most important to me, i've lost about 6mpg which equates to about 100+miles less on a tank. Known problems are the differential leak as mentioned b4, no back o2sensor, downstream of the converter, and lastly maybe its as simple as switching the spark plugs, since I just had them changed prior to switch, he was gonna put them into the swapped engine but saw that it had titanium plugs that he said should b good 4 100,000mi. I'm hoping it's as simple as the plugs, maybe gapped wrong, but i'd appreciate any imput you may have, could plugs really affect my mpg like that?