rear disc from grand cherokee on a 8 3/4

You might be able to but I think you would have to eliminate the fifth bolt, the jeep only has 4 because that is where the wheel cylinder is for the e brake. Not sure what that would do to the strength of the axle flange as far as holding it in on the 8 3/4.
We ran in to another problem the axle dosn't stick out far enough to allow propper alighnment for the rotor to the caliper.
We would had to have added a backing plate to the rotor. Looks to be around 3/4 of an inch.

We have a 8 1/4 sitting in the shop so we dismantled it and tried the jeep disc's on it. They seem to be a direct bolt on and everything lines up good.

So for now we're going to forget about putting them on the 8 3/4 and put them on my son-in-laws 8 1/4 under is dart.
We'll be doing that this friday so i'll take pictures and post here.