Rear-end "clunk" at speed

[Pete]

Hello all.  Seemingly awesome site.  This is my first post here, glad to have been referred.  On to my plea for assistance.

 

I drive a '96 Tahoe, 4wd, Vortec 5.7L, auto-tranny, that has about 115K miles on it.  I'm the original owner, and the only modification is a K&N filter.  Recently, I have noticed a slight "clunk," for lack of a better description, when cruising around 60mph, with 1800rpm showing on the tach.  It seems to come from the rear end, like it is slipping or something, similar to what it would feel like if the tires broke loose for just a split second and caught traction again.  I have only noticed this happening when it is in overdrive.  All else seems to be fine.

 

Any ideas as to what's going on with this thing?

 

Thanks.

[Colorado Kid]

Could be Torque Converter (TC) unlocking and locking back up.  Check connections on wiring leading to Transmission.  Also check brake light switch, as this can cause intermittant TC spasms if it's causing the brake lights to flash intermittantly.  All this is assuming that your left foot is nowhere near the brake pedal when you are cruising. ;)

[Pete]

Thanks, Kid!  I'll do those checks.