I have a 99 F150 Lariat with the 5.4.. transmission acts like it’s in neutral in every gear.. Also have a 98 F150 Lariat with the 4.6… Would the transmission from the 4.6 fit on the 5.4?

 

If the 99 is a 4R70W, the 98 transmission will fit as long as they both have the same drive system (2wd or 4wd). If the 99 is a E4OD/4R100, not interchangeable at all. The 4.6 is a 4R70W for sure, the 5.4 used both. Use a 5.4 torque converter.

 

how do I tell which one I have?

 

Check the TRANS code on the door jamb sticker. 4R70W is U, E4OD/4R100 is E. You can also look at the pan, the 4R70W is roughly rectangular and symmetrical, the E4OD/4R100 pan looks like the state of Utah, it's missing a corner.