Repair on my Dodge Nitro 2011 Heat

$4400 is way way way over priced for a second hand 3.7l engine, with no guarantee of the 98k mileage
a bill for $8k i would insist on plenty of warranty with the engine
you could buy another Nitro and still have plenty of change,

pretty sure you can file a complaint to some official body, and they will help get some money back,
most common thing that does not get done correctly after an engine swap is they never bleed the cooling system correctly, so take it for a GOOD drive and make sure the temp gauge and heater are all working correctly,
don't sign anything unless you are happy with what they have done,

and don't beat yourself up about the situation, because many of us have been put in the exact same situation, once the garage has the car and they say sorry but your car died, what do you do, scap it or have it fixed, most of us have our pride and joys fixed and we over paid for the repairs,
fingers crossed everything is ok when you pick it up, and they have done a good job,