You can unplug the transmission and that will leave you with max line pressure, Reverse, 3rd take-off in D or 3 range OR 2nd take-off in 2 or 1 ranges. If it is not engaging then there is an internal problem or it is very low on fluid. As Bill suggest- MAKE SURE there is a filter in the trans! If missing it would cause slippage across the board but strong engagement in Reverse says the Forward band is not holding. "IF" a Shift Kit was installed- then it sure sounds like two of the metal tubes were accidentally swapped and when this happens you have Forward Band fluid pressure going to your geartrain, and rear lube fluid trying to partially apply the Forward band and it is likely smoked already if that is the case and it has been driven.
