2000 blazer have to manually shift
 

why could be wrong? i have to start in 1st or 2nd gear then at 20mph i can shift to 3rd gear and at 35mph i can shift to drive...if i press on the gas to hard it just revs...at a stop drive and 3rd gear just revs...dropped pan and changed the shift solenoids and filter and pan seal and refilled with 5qts of tranny fliud...thanks