Not simple vibration if rpm is1500 - Trooper Isuzu Cars & Trucks

have to decide if the vibration is engine or transmission sourced
Is the vibration there in neutral at the rpm of 55 mph or better Taking that at 55mph you would be in over drive and the rpm would be around the 2600 mark
hold the rpm steady and see if the vibration is apparent
raise the rpm to see if it comes in
if it does , it is an engine problem and probably a balance shaft timing out
If not then it is a transmission problem and as it would be in over drive and the indications would be a failing bearing in the rear of the transmission of transfer case

does it have the same acceleration at that speed, if it does not accelerate so well it is called a flat spot and will almost certainly need a good mechanic to sort it out
If acceleration remains as good in this range as through the rest of the rev ange add detail and please resubmit

Are you sure they hooked the crankshaft dampener back on?
http://www.dealsinapinch.com/A_D8NN6316AC_p/a-d8nn6316ac.htm
Does the vibration happen at all rpm's or just a certain range? The Flywheel could not be centered exactly (or even reversed) or not balanced

Sounds like you need new engine mounts

Sorry for the late reply but, no, it is not related to a loose heat shield. It is a louder rattling sound more like a loose bearing vibrating inside either the transmission or the clutch assembly. Only between 1600 and 1900 rpm. and it is not present under ou over these rpm. The rattle desappear when I lock the 4X4. Dealer is out of solution!!

check for problem drive line "U" joints or cv joints

Timing or a balance issue. Make sure the harmonic balancer it tight to the crank. Also make sure the clutch and pressure plate are tight.

It will be muck or something on the front wheels . Clean them inside and out and then have them balanced.

The vibration at that high rpm is most likely coming from the torque convertor and if a manual transmission , then clutch issues.

try by replacing plug wires and spark plugs, also add some fuel injector cleaner to fuel tank, i will recomend techron brand or si1 from red line but you can use almost any one