If there is a vibration or noise at highway speed, it could be from the frontdrive shaft and may require replacement of the double cardan joint. A double cardan joint is a type of driveshaft joint that uses two universal joints.
1996 Ford F-150 Problem Reports
RepairPal Verified 1996 Ford F-150 Problem Reports
The engine oil pan gasket can develop a leak. Leaking oil may accumulate on the exhaust system resulting in a burning smell. Replacing the gasket should correct this leak.
Tubes for the EGR system (exhaust gas recirculation) can break, causing an exhaust leak and noise under the hood.
If a noise from the rear end is heard when going around turns or after driving at highway speed for an extended period, there is an updated differential clutch kit available.