I'm not 100% sure, but I believe it is a clearance issue with the front axle. I think that the sump on the 2.8 sits back furter. It's not the depth that's the issue, it's the setback on the pan.

There's a lot of people on here that have done the swap and I'm sure someone who knows for sure why you have to swap will chime in.

1995 Trooper LS auto 3.2 DOHC /w SOHC intake
1989 Trooper 2.6 auto
1989 I-Mark RS DOHC 1.6
1991 Stylus XS DOHC 1.8