Had the same starter issues with my 84 Bronco 2 XLT....the spacer plate on mine was broken, but that didn't really seem to be the issue. The starter itself just would never stay in position no matter how tight I ran the bolts in. I finally drilled a through hole on I think its the top hole and put a washer, lock washer, and lock nut on it and it helped a little, but after about 6-7 starting sequences it was right back to grinding away and not engaging again. Tried a couple different starters from the 4L, and other 2.8L engines and still had the same problems. Finally removed the spacer plate around the starter area and it seemed to work a little better, but still had the starter sliding out of position just enough to have the thing grinding most of the time. I finally had enough of it and scrapped the thing, but it had a lot of body rot around the floor pans, door sills, etc....it had been driven through a lot of the salt belt in Utah during the winter so it pretty much rotted to pieces, and I don't think that had anything to do with the starter issues as it seems to be quite the common problem.
I've heard others mention different starter gears 9 VS 10 teeth, never got around to trying to find a different starter with 9 teeth, and given my situation where the starter kept sliding out of position there was more of an issue somewhere else than gear teeth...I think the bolt holes for the starter bolts on the transmission housing were wallered out, so with the torque from the starter even cranking the bolts down as much as I could wasn't enough to keep the starter from backing away over a few starting sequences.
Sadly that thing ran and drove great, just the body was rotting away around the engine.
My 88 B2 was an automatic, and luckily didn't have starter issues with it other than when I initially bought it, the starter was kind of hit and miss so it was replaced during the transmission rebuild process and never had a problem with it afterwards.