Hello Tom --

To begin, I would not leave transmissions aside due to the fact that they are unique to the diesels (and in my experience, identical save for the bellhousing anyway).

Also, I am not sure whether the 1987 chassis should be grouped with the 86 and earlier or 88 and newer models. I've generally heard 88-91 are similar, with removable crossmembers etc., with the front suspension changing somewhere in 89 (likely irrelevant to this discussion).

I think the thing to do might be to determine which frames are similar.

I can relay what I've heard re: 3.1/3.0 diesels: the 3.1 (4JG2) supposedly shares the same block with the 2.8, just with a larger bore. The bellhousings are supposedly the same. I've also heard that the 3.0 (4JX1 I believe) block/bellhousing is the same. No concrete proof of either.

I've also heard the 3.1 is to be avoided. The 3.0 is the most modern diesel of those you listed.

So, it would seem that the 3.1/3.0 would also drop into the 88-91 chassis. I think the 3.1 was fitted to the 92-98 (?) style Trooper, whether the gas/diesel mounts are the same is not known to me. Past experience would seem to indicate that the frame mounts would be the same.