Expert: CAJUNSC/RAMBLER390 - 7/29/2009

Question
Hello,
Rear rear is clunking into gear. Ujoints are fine, lots of play when feathering throttle off slow. Figuring on rear gear replacement along with front diff the same with a lowering of ratio down to 3.08 ratio. Will this work on a truck that is strictly used for 70-80 mph commuting on the Garden State Parkway.
These rears and gears have 197500 miles on them. What problems would I endure when trying to become more green.

Answer
Hi
First, my personal opinion is UNLESS your differential is making any noises (roar, whine, whirr, etc...) I would make DANG sure the ring/pinion is your culprit.
GM's have historically made serious clunks between shifter gear changes (park to rev, rev to drive, park to drive, drive to rev) MOST often the "play" or slack is from between the gears meshing. It gets progressivly worse over time. It also get double worse in 4x4, due the the transfer case/front differential. Not saying the ring/pinnion are bad, but unless you have the $$$$.$$ to throw at it I would have it throughly checked. The Drive shaft to trans output shaft splines are REALLY notorious for makeing gear shift clunks, even on 2009 models. You MAY opt for replacement of the driveshaft input shaft or the trans. output shaft (or both).
I said all that if the clunk is the only concern.
As for the gears...
The 3.08:1 will work better for the interstate driving. A numerically higher ratio will cause the same tire travel but at higher RPM. For example, a 3.42:1 ratio will cause the same tire travel but at higher RPM than a 3.08. You will loose acceleration "off the line" with the numerically lower gears(eg 3.08:1). Higher ratios = quicker acceleration. Lower = better "top end"
The avalibility and cost will be issues. Rear diff parts are not cheap. (at least from GM.) Check online sources, aftermarket, etc... As far as replacement, as long as a good reputable shop that can do or has done re-gearing is preferred.
Good luck
Pawl