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Q: clunk on 1999 Mazda B4000

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
bought a 1997 B4000 with 120k miles. Rear end made grinding noise when moving from standstill, sepecially if turning sharply during startup. Also made clunk when stepping on gas and sometimes when breaking when the nearly stopped. Used car dealer said someone put in gear oil without proper friction additive. He put some in and grinding noise has gone away (except on very rare occasions), but clunk remains. Is this clunk indicating differental repair is needed? Dealer promised to fix the noise if it returned.
My guess is that there may be damage or wear in the limited slip mechanism, expecially if the additive helped. May be too late, may need the oil drained and fresh oil used with the proper type for limited slip diff's. Check with the Mazda or Ford dealer for proper type if it's not in your owner's book. If that dosn't fix it, consider guaranteed good used parts to economize on an 11 yr old truck. Oh yeah, make sure the u-joints are OK and driveshaft isn't diamaged, look for loose mountings, lugnuts, noise may not be from inside diff. Check for loose tailshaft bushing in trans by grabbing (with trans in neutral and brake on, wheels chocked) front end of driveshaft and pushing side to side, up and down for looseness in bushing.
Thanks. I'll check out as you suggest. On diff, the dealer still claims he will fix by replacing "parts" but he doesn't have them yet (2 weeks).
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