Anyone know why my truck wont shift?

i have a 89 bronco full size 4.9l that had a rebuilt transmission about a year ago and it threw a rod so i bought a beat up 91 f150 4.9l and did a engine swap. so i got it running the other day checked the fluids and started it. but now the bronco wont shift out of first unless i put it in 2nd (its automatic) and if i put it back in drive then itll stay in 2nd. i'v checked the transmission fluids ther fine but thers a small leak in the t lines not major. and the transmission was fine befor the engine went bad. any sujestions?