It's actually a Chevvy engine, not GMC. GMC did a better job...
Look for a donor for the swap - S-series with the 3.1L or 3.4L will be a drop-in for the whole powertrain (1988-up THM700R4 or 4L60. The 4L60E should be doable with some effort...) and will pick up an overdrive gear as well.
The F-body (Camaro/Firebird) with the 3.1L/3.4L will be a drop-in as well - but the engine only.
FWD variants will only bolt in if there is an unused starter motor mounting pad on the back of the engine block (as mounted.) Dedicated FWD engines have the starter on the wrong side of the block.
The THM700R4/4L60 will require some small fabrication to move the crossmember and make it work, but I think it's worth it. An effective THM700R4/4L60 is a better transmission than the "Baby 904" that AMC used, and the overdrive can pay for the swap with a lot of highway driving. The transfer case used behind the THM700R4/4L60 is the GM/Chevvy version of the NP231, with a wider chain. You'll probably have to do some fabrication to make the transfer case shift linkage work as well - but the transmission shifter should be easy.
The 2.8L V6 was a frickin' dog in terms of power, and leaked like a seive at the rear intake/cylinder head/valve cover junction. I had one in a pax car, and I never could get the damned thing to seal! I just topped up the oil as required, and changed the filter every six months (the engine oil pretty well changed itself.) Next car I had had the 3.1L V6 in it, had a 300# greater kerb weight, and the same transmission & final drive ratio. Ran like a scalded hog, didn't leak, and was a quite reliable vehicle.
The point of all this? I say to avoid the 2.8L, unless you're going to plan a swap at the same time. The use of the 2.8L was an interim measure by AMC - to get the XJ going with a "high performance engine option" (it wasn't) until they had the 242ci I6 ready for market. Considering that the 150ci I4 was comparable to the 173ci V6 in both horsepower and torque output, it wasn't very "high performance" - they'd have been better served using, say the odd-fire 231ci V6 instead...