Are the doors and windshield interchangeable between a 1990 and 1991 Isuzu Trooper?

Yes, the doors and windshield interchange as follows: 1984-1991 windshield interchangeable. 1984-1987 2 door LWB doors interchangeable 1984-1987 4 door doors interchangeable 1988-1991 4 door doors interchangeable 1989 SWB 2-door doors interchangeable with 1988-1991 4-door front doors and rear barn doors. note: pre-1988 4-door doors will fit 1988+ mostly, but the lock mechanism has a different retaining hole. but the newer locks can be re-keyed to match the older keys, and vice versa. you just can't put the whole lock from one into the other. There were also 5 distinct hinge redesigns which make door fitment slightly different with different years - for example the hinges from a 1986 would swing the doors too wide on a 1991 and allow little kinks from the interfering sheet metal.