Is it a 4x4? if so could be slack in the transfer case. Is there any noise from the rear when driving? I had a sort of whining noise in my 93's 8.8" and turned out to be that the pinion ended up getting pushed into the axle housing an over a several thousand miles, it had ground out about a 1/4" from the axle's housing. The noise was noticeable under loads or when decelerating, but got worse and worse until it did it all the time. Then one day the floor started shaking as i was cruising at about 45mph and pulled over to find that the rear end was toast. I had someone change the tranny from drive to reverse while holding the brake. I watched the driveshaft at the diff turn almost 3/4 a turn and the yoke would move in and out of the axles about 2" I ended up taking the shaft off and ran the truck in 4wd (converted it to a FWD truck temporarily) until I got the parts to fix it. The rear axle was entirely coated with metal filings inside. All the seals were shot, and the pinion bearings were torn to pieces. After cleaning up the mess I installed a new ring and pinion set and a limited slip diff from a 2003 cobra mustang which I got off of ebay all brand new for $300. Bearings and seal set cost about $180 at a local drivetrain shop. Did all the work in about 20 hours straight. I flushed the oil several times after that to make sure any loose filings were removed. I've put on another 50,000 miles since then and it is still nice and tight.

I guess the moral of the story is that all the noises you hear are telling you something, and it might take some hunting to find out what it is, or just wait till it breaks like i did! The bronco now has 273,000 miles on it, which is quite a bit more than your 89. I would have someone shift the tranny between forward and reverse and take a look at the entire drive line underneath and look for a play and listen for noises. You may not hear it from inside unless there is a large load, but it may be more noticeable up close. Just make sure they keep their foot on the brake.