First of all, I finally got around to installing my SR5 gauge cluster- thanks to Stock87 for the webpage writeup, everything went in without any unexpected problems.

Also, I just put on new 30x9.5 Kumho ATs. My truck sees mostly highway driving, all at around 6500' elevation, with many slightly higher elevation mountain passes on a regular basis. With the 4.10s and the new tires (old tires were 235/75) I notice a loss of power, but not too bad. I wouldn't really even worry about it, except that my rear end sounds like it is in need of replacement anyway. At speeds of 65 and over, it hums louder than the engine noise. I have changed the oil recently, and oddly, there were no metal bits in there whatsoever. So, to add to the 18 billion posts on what to do regarding gearing, I'm looking for recommendations: should I just find someone selling a used 4.10 third (they regularly show up on pirate or elsewhere for $30-50), and swap that in, or would it be worth it to go to one of the local yards and find a pair of 4.56 diffs? I don't ever plan to go bigger than 31" tires, and chances are I'll stay with 30", so I don't think I need 4.88 gears. But, I don't know if it's worth the extra $400 or so that it will probably cost me to get the 4.56 diffs.

Any advice?

if your sticking to 30"ish tires, my advice would be.
go find a junkyard truck with an auto trans. the auto's came with 4.30's and that should be perfect for 30-31" tires with a stick.
you'll just need the 3rd's so it shouldn't be to pricey, that'd be my choice...........Karl

I bet you have a bad u-joint on your rear driveshaft which is causing the humming. Check that out first.

I'm with little joe. Check the u-joints. How many miles on them? Are they lubed? Is there any vibration in the rear of the truck when you drive?
Replace the cheap stuff first!
thefreq <img src="/forums/images/graemlins/cheers.gif" alt="" />

Yep, I didn't even think of that. Thanks for the help guys. So, how would I check for u-joint wear, other than obvious things such as grabbing the driveshaft and seeing if there's play, or seeing if it's lubed at all? I'm gonna do a search right now, but any advice is appreciated.

One thing though: There really isn't any vibration at all, at any speed. I mean, things vibrate and shake somewhat at speeds over 60, but no more than you'd expect on an 18 year old truck with 187k.

The "humming" does seem to be worse when coasting. If I'm coasting down hill, it sounds as loud as the engine does at 3500 rpm, and very similar in tone, but if I put it in gear and give it some gas, the humming quiets down slightly... not much, but enough to notice it.

I just got under my truck and pulled on the driveshaft... and sure enough, it's loose at the rear u-joint. It was hard to tell exactly how much it was moving, but I'd guess it had ~.020-.030" of play. Thanks again for the tip guys, it does seem like that was the problem.

Put the new u-joint in last night. Took about 3 hours. I had a hard time getting the ends of the driveshaft assembly off the of their respective mating flanges, until I took a chisel/wedge and drove it into the only gap between the yoke and the flange, and then each end popped off right away. Then, I used the advice of the u-joint write up on the jeep section of 4x4wire (u-joint change) and got the first couple caps knocked out no problem... but then I had the u-joint still in the small end yoke and had no way to get it out. I should have knocked out the caps to get the small yoke off first, then I would have still had the driveshaft to aid in holding the assembly while the last two joints caps were knocked out. But, I got it all apart.

So, I started pounding in the first two caps of the new joint, but no matter what I did I could not get it to go together enough to get both c-clips on... I had about .040" more to go, and I could not get the caps on any farther. So, I took it all apart again, only to find that one of the needle bearings had become dislodged and was sitting in the bottom of the cap, resulting in the ~.040" gap that I couldn;t get rid of. Anyway, I packed a bunch more lube into the caps, and made extra sure to slide it all together while not letting the bearings move, and then it all worked fine.

End result is a much quieter and smoother truck at highway speeds. I can actually make out individual cars in my rear view mirrors on the freeway now, since the vibration is gone.

Thanks again for the tips guys.