I've got a 1999 F-150 4X4 with 125k miles. When shifting from P, R, or D, there is a very noticeable clunk. When shifting, the motor will rev, then once the transmission is in gear, the rpms will drop momentarily. Once the trans is in gear for a second or two, the truck will idle normally. I will be changing the trans fluid this week sometime but I doubt that will help the clunk much. Also, u joints have recently been replaced. Could this also be a problem with the rear end?

 

Sounds like you're getting "flare shifts" on the tranny. The clunk might be the rear or the U-joints or it could the the transmission, only a hands-on inspection would tell for certain.

There are several different common failures in the 4R70 that can cause either flare shifts or harsh engagements.

I'd suggest that you take it to a reputable transmission shop for diagnosis. Get referrrals from friens, family members, or work associates. The shop should be able to take pressure readings and get a good idea of what is worn in the transmission. Based on your sketchy description, I have a couple of ideas, but do get it checked by someone who is knowledgeable about this particular transmission.

Steve