Re: 99 Laredo cherokee, oil pressure gauge at 0-
Happens a lot that new sending units are faulty, so i would check an other one first. You are sure that the connector is fitted?
If the engine runs, does it make a different noise then before (more mechanical noise due lifter tick for example could be an indication of real low oil pressure.
If the oil is all up the stick, then there is something not ok. You did clean the dipstick first before the measurement? You are sure its the right dipstick and not the one from the transmission?
Too high level could be caused by coolant in the oil, is there any sign of white stuff or other strange gunk on the dipstick?