U-Joint problem? on 1994 F-150?

Whenever I am in the low rpms at exactly 45-48mph and 60-65mph my truck shakes but when I give it gas and it downshifts it seems to come out of it but it still stumbles under heavy acceleration, could this be a bad u-joint? It doesn't make sense to me because I just replaced a u-joint, plug wires and about 2500 miles ago, and after that it ran great, and I just replaced the fuel filter about 200 miles ago and I also put a fuel additive into the gas tanks thinking that a injector was plugged but that did nothing, but I am pretty sure it was only the rear u-joint that was replaced.