Justin Rountree

Ok guys, first off-- I am brand new to the forum here, so thanks in advance for any help you guys can offer. I have a 1998 Chevy Z71 Ext Cab with 5.7L v8 and 4L60E automatic transmission. I bought it about 2k miles ago when it had 191k miles on it. It just rolled over 193k miles. Over the past several weeks it suffered a gradual transmission failure that ultimately resulted in my getting the transmission rebuilt. I just went and picked it up from the trans shop today and after test driving noticed a problem... I'm not too worried because it has a 1 year warranty and I already let the shop manager know and he's going to test drive it with me to troubleshoot the issue tomorrow. Anyway... I was headed home from the shop today and started noticing a vibration. Now before you guys start ridiculing and asking me why I didn't test drive it before leaving the shop--I did. The problem is there is so much stop and go traffic I couldn't really get it up to 60-70 mph. I felt it shift through the gears fine, went in reverse and everything felt fine. But after I got it on a long stretch of road I started noticing this pulsing vibration in OD. Like I said the transmission is not slipping at all, but as soon as it shifts into overdrive in predictable fashion it started to vibrate--not a constant vibration, it has an almost rhythmic vibration. And it only does it when I have my foot on the gas. If I let off the gas, or put in neutral it does not do it. It also does not do it in park or lower speeds. Also I tried putting in 3rd gear and it doesn't do it at the same speed. So to recap, above 50 mph in overdrive, foot on gas = rhythmic vibration. No vibration otherwise... At least I thought. I manually shifted to 3rd gear and it stopped and I drove it all the way to 60-65 mph in 3rd with no issues... Kept it out of OD driving around and thought for sure it only did the vibrating in OD but then noticed if I'm going up hill in 3rd gear and the RPMs around 1500, it will do the pulsing vibrations although not as noticeable as it would if it was in OD. I'm expecting someone out there to suggest I have an unbalanced wheel or tire, or that I should check my plug wires or that I have a bad u joint. Probably not any of those things, so please resist the temptation to suggest that. All I know is, the vibration was not there before I had the transmission rebuilt. I have read around and can't seem to find alot of descriptions on the web that mimic my issue. Some have mentioned something called a "chatter" between 4th and lockup related to torque converter, but I don't think this is the same thing. There doesn't seem to be a hesitation with the TC lockup. It gets into high gear just fine, but when it does it goes into this rhythmic vibration, if I let off the gas it stops, if I manually shift into 3rd gear it stops. If I press on the brake it stops... And then resumes again when I depress the accelerator. So I have a few ideas and wanted to know what you guys thought. Possibilities are as follows: Bad torque converter (even though it was replaced with a remanufactured one when it was rebuilt); bad TCC; bad valve body?... Maybe even a misaligned/unbalanced drive shaft--although that seems unlikely given that it doesn't do it at the same 50+ mph speed but in a lower gear. I am at a loss... Any thoughts?