2 Answers

Somethings that could cause issues in your truck's transmission:

1989-2003 Models
The 1989 to 2003 F-150s also have been known to have transmission fluid leak into the transfer case. In addition, the torque converter dust plug can fall out, requiring a sealant to keep it in place.

Shot Torque converter
Incorrect use of towing can cause significant damage to the automatic transmission. Rocking the F-150 back and forth by quickly shifting from reserve to forward multiple times to gain traction causes enormous stress on the torque converter. Such abuse may result in the owner hearing a loud "bang" followed by the truck not moving forward. If an internal examination reveals metal splinters, the torque converter needs to be replaced.

Read more: Ford F-150 Transmission Problems | eHow.com http://www.ehow.com/about_5569232_ford-transmission-problems.html#ixzz234nJ7SO6 

As already recommended, take your truck to a transmission shop and have it checked. The inspection is free and will at least let you know what the problem is. You can decide from there if you can fix it yourself. 

You need to take your car to a car mechanic, who will check what the problem is and how to fix it.