New to the Ford corner of the ring guys, so cut me some slack. Just bought a 1998 F-150 reg cab ext bed with the 4.6 and an auto trans and I'm trying to get my bearings. Since I've had it, Higher gear + Low RPM's = Vibration. The previous owner checked what I think of as the usual culprits: changed plugs and wires, had the computer scanned, and even had the injectors flushed. Looking for any other ideas y'all might have before I turn it in to the shop. Thanks.

FYI: No check engine light is on, and the problem really shows up bad under load right before the trans downshifts.

check motor and tranny mounts. It could be the torque convertor not wanting to disengage from lock up.

Does it feel like it's running over rumble strips, or something similar?

This *sounds* like a classic case of Torque Converter shudder. It can be easily cured with an ATF change unsing the proper fluid (Mercon V ATF). There is a tech article at www.crownvic.net/tech/transservice.htm

I wish y'all would've been right. Started feeling a vibration at idle today, as well as heard a slight knocking in time with the rpm's. Got out to check, and sure enough, dang thing's shakin' and knockin like I thought. Please tell me it isn't time to fork out some cash, because if that's the case I've got an F150 for sale.