99 Tahoe miss,vibration

99 Tahoe 2dr 4wd 69k 4" lift new 305 70 R16.
Since I've owned this vehicle it has had this vibration between 50-55mph no faster or slower. Noticed my tansfercase had a leak, nothing too crazy so I thought is was either the front or rear snout gaskets going bad or the case halfs were leaking. Took it out and noticed there was a hole the size of a pin head on the back case half. I guess with these autotrack transfers cases the internal oil pump wears a hole in your case eventually and GM hasn't fixed this issue & when you buy a remanufactured one from GM for $1200 plus it doesnt have this kit that is offered by some tranny shops that prevents this from happening in the future. If you have tansfercase NP246 these case halfs are very hard to find. There is no fixing the hole since the case is magnesium I guess you can't just weld it. Long story short transfer case caused my tranny to go bad and needed a complete rebuild as well. Now I still have same vibration but between 40-50mph in overdrive only and it either surges 100-200rpm or drops 100-200rpm. Any ideas? Already replaced TPS, cap, rotor, wires, plugs, fuel filter. Vehicle runs perfect other than that. Clean starts, idles great, etc... Any ideas would be awesome.

Update....While cruising I had my diagnostic tool plugged in that gives real time data. 40-50mph when the truck either surges 100-200 rpm or drops 100-200rpm my timing retards 10 to 15 degress. Sometimes even 25 degrees. No misfires, fuel pressure is fine. Maybe a faulty distributor?

are you sure the case is magnesium and not aluminum?( more likely) maybe check with a local trans .shop to make sure . if it's aluminum have it welded. as far as the other problem, have you checked out the camshaft position sensor & the crankshaft position sensors . i would suspect one of them being at fault and causing the engine timing to retard at speed. that's where i would start looking.. exwldr....:sunglasse

Hello, Have you found out what the problem was with your truck yet?

The vibration at 50-55mph sounds like the same issue I'm having.
Back in May when I first noticed the problem I took it in for a check up, got a tune-up - spark plugs and wires replaced, the fuel injection cleaned and the distributor cap, which was loose and cracked, replaced. After all that the truck drove smoothly with no problem. About 100 miles (I change based in mileage, I've been putting on a lot of miles)from its next oil change it started vibrating again, and my "check gauges" light came on, I checked the oil and it was 3 quarts low (strange because I've never had to add oil to any car between changes before) , I added some oil and it started running fine again. This became a pattern, vibrating and low oil before the next change is due, add oil vibration stops.
So when the car started vibing again I figure its ready for another change (since vibrating always happens around the same time as oil is due) so I took it in and got it changed, problem is this time the vibrating hasn't stopped.
I will be taking it in for service and hopefully will find out what's wrong without being raked over the coals by the mechanic and the warranty company. Just want some feedback before I go in so I don't go in blind.

99 Tahoe 2dr 4wd 69k 4" lift new 305 70 R16.
Since I've owned this vehicle it has had this vibration between 50-55mph no faster or slower. Noticed my tansfercase had a leak, nothing too crazy so I thought is was either the front or rear snout gaskets going bad or the case halfs were leaking. Took it out and noticed there was a hole the size of a pin head on the back case half. I guess with these autotrack transfers cases the internal oil pump wears a hole in your case eventually and GM hasn't fixed this issue & when you buy a remanufactured one from GM for $1200 plus it doesnt have this kit that is offered by some tranny shops that prevents this from happening in the future. If you have tansfercase NP246 these case halfs are very hard to find. There is no fixing the hole since the case is magnesium I guess you can't just weld it. Long story short transfer case caused my tranny to go bad and needed a complete rebuild as well. Now I still have same vibration but between 40-50mph in overdrive only and it either surges 100-200rpm or drops 100-200rpm. Any ideas? Already replaced TPS, cap, rotor, wires, plugs, fuel filter. Vehicle runs perfect other than that. Clean starts, idles great, etc... Any ideas would be awesome.

Replaced the crankshaft position sensor and camshaft position senor. The rpms no longer drop 100-200 rpm anymore. They only surge 100-200 rpm. Plugged in my diagnostic tool again and drove around. The timing now advances 15-25 degress when the vehicle surges. At this time I have one other option and thats to replace the knock sensor. Fuel pressure is where it needs to be and there are no misses at all while driving or at idle. If not the knock sensor any other ideas?

IENCYST....Sounds like your issue is differenet from mine. If your 3 qts low every oil change that means you have a pretty big leak somewhere or your burning a lot of oil. Does you truck puff blue smoke when it starts? Your vibration might be your crankshaft. When running on 2qts of oil maybe the vibration is your internals not getting enough lube causing your engine to run rough. Be careful not to scortch the crank bearings.