You sure it's not a u-joint that's going bad?
Also, what type of limited slip? Does he have Ford's "friction modifier" additive in it? If so, the stock ones shouldn't make any "clunking" sounds.
Ok, here it is more information, door code for axle is "H" the Trans code is "5". The axle tag code is WFD-F/981 and it is the origanal Traction-Loc rearend that came in the car. The car is a 1968 Shelby gt500kr.Thanks for the info so far. I have not driven this car in 15 years and am having problems remembering all the little things about it. I remember a clunk but it sounds louder now or I'm just older and less forgiving. Joker
WFD-F is for a 3.50 geared limited slip type rear, but tag wouldn't specify type. My guess if a 68 would still be an equa loc, just had an original N case 68 4.30 drag packed geared rear with original tag that I sold, it had the equa loc and it was an early June built 68.5 CJ fastback.Earliest trac loc unit I have ever seen was the C9 part number, which with Ford doesn't really mean much at times.The 981 (hopefully )is code for a 68 9 inch Mustang housing type (I don't yet have all specific but it should match build sheet under "axle" ), there should be a date code on tag itself in form of a number/letter/letter (ie. 8FA= first week of June 1968). The build sheets I have are for 390 cars with 9 inch 67/68 and 67/68 8 inch cars- they show 996 for 67/68 9 inch housing type and 930 for 8 inch. On build sheet the three digit number will be displayed with a model year digit before and a suffix type afterwards (I don't have all the facts yet on suffix's). But a 67 build sheet for a nine inch in a 390 shows 7996A, while a 68 shows 8996A etc.. I add this info because my source shows the WFD-F as a 69-71 only code, not 68, but I know there were exceptions.
Are you sure it's the original rearend for that car? I'm thinking the "981" you see is actually "9 B1." This would translate to a build date of Feb 1, 1969. This would appear to make sense since the axle code (WFD-F) is supposedly for 69-71 Mustangs. If that's the case, it should have a traction lok in it. I don't when the first traction loks were used, but the earliest one I've ever found was in a 68 station wagon. I remember it because it was an odd one. Nine inch, 3.25 gears, 31 spline TWO-PINION traction lok.