First post, looks like a great site!
1998 F150, 4.6L auto, 118k

Father in law had been driving my truck for about a month, I got it back and
now it's acting up.

Here's what it is doing

If you leave it in 2wd, there is a slight vibration from the rear.

I am thinking u-joints, as I have heard them squeak a little.

No big deal there...but here's where it get's strange.

Driving last night, I wanted to check to see engaging 4wd would change
the vibration.

It was raining/slush on road. I clicked into 4HI (elec. trans. case), and
as I sped up from 40-50mph there was a very significant vibration,
and it seemed that I lost some power...just seemed to drag down more than
anything.
4wd was working though.

So I switched it back to 2wd, and it did not immediately disengage.
When I slowed down to about 20 or so, it did finally disengage with quite
a clunk. It never really "clunked" before.

So, being the curious type, I waited a bit, then re-engaged 4wd.
Same symptoms as before.
I switched back to 2wd, this time it would not disengage.
I stopped. I put it in reverse and backed up.
Still in 4wd.

I went forward, back up to speed, still engaged.

Finally stopped, put it in neutral and shifted to 4LO, then back to 4HI
and finally 2wd. Did this twice, then heard it disengage.
Started out, and was fine, still with the slight vibration from the rear.

I did not try the 4wd again.

I have not climbed under the truck yet, just did not have time....but
wondered if anyone else has seen/heard this....and where I should start looking.

Thanks in advance!

 

4wd puts a bit of a load on my truck makes it feel slower and noiser that 2wd. more fricton in 4wd...do your ujoints. i just had mine done and you wouldnt believe the diferance in the truck.