Learjockey1 Being a short drive shaft, there is going to be a little vibration because you don't have the room for a dampener ring on the shaft. Have to remember that our motors are off centered and it is important to have the right angles from the trans to the driveshaft and driveshaft to the rearend. We also have a little right to left off center too. If you got your drive shaft from ERA they have grease fittings and only need a couple of pumps to remove any play in the universal cups. I use
Amsoil hi temp grease in all my joints, upper and lower ball joints, inner and outter tie rod ends, inner and outter drive shafts between carrier and outter hub. Some hubs have grease nipples for the hub bearings, mine do.
You do need some play at the slip yoke. The problem may not be the yoke but the yoke bushing in the tail housing of the trans. Do you have a leak, wet spot, or any spray looking around this area? If yes, replace bushing and seal. You can add a little special grease inside the yoke that doesn't break down from the fluid. I will have to get you a GM number for it, comes in a kit. If you have a vibration at a certain rpm range, it could be any thing in the drive train from clutch to driveshaft to rearend yoke without a weight or may have a wobble. If it is really bad, take the drive shaft and have the balance checked. If it's not, than start with removal of the drive shaft and see if it happens in the rpm range without the rearend hooked to the motor and trans. If yes, start at shaft and then go to rearend, if not start at clutch and remove and go from there forward.
Do you have an ATI dampener on the motor?? I have had issues with it at times until the fluid warms in it and the weight moves.
As for length on driveshaft, IMO I like them with alot of spline in the yoke. I leave about 1" until bottomed out on tail shaft.
How many miles and is there any abuse done to the drive train? Where did the trans come from? It may be a simple as just needing a bushing and seal. Do basics first. Last thought and you need to be REAL careful. I don't recomend this to most garage mechanics because of safety.
Put the car up on jack stands, For the rearend have them as far out as possible to hold the weight of the car.
Make sure the tires don't rub
Have the tunnel out of the car
Start up the motor and run the car till you get the vibration
good light and a shape set of eyes will show it any thing LOOKS out of true spin. Start with that section for the vibrations. Wheels and tires get flat spots and have vibrations also after sitting on concrete floors for a couple of months. May need a balance for the tires. .5 oz could cause a vibration at a certain rpm range and not be felt at a high rpm.
If you can't find, send the car back to ERA. Let doug fix the problem. He's a top mechanic for finding these kind of problems and fixing them.
GM has a EVA machine that gets hooked to the chassic of the car and shows Hertz and helps isolate where the vibration is coming from. Doesn't work so well on a cobra. Good luck Rick L.