97 Ford F-150

There is no relay but there is a fuse in the junction block, #23 10 amp should be hot at all times, the fuse runs to both the 2wd and 4wd solenoids and to the gem module, besides that there is a vacuum reservoir in the engine compartment and a check valve in between there and the engine, the only other component involved is an actuator on the transfer case, I would start at the fuse and work from there checking vacuum to the units making sure you have vacuum from the engine to the res, if all else fails it may be the gem module, but I always say start simple and check everything three times , good luck on the hunt

Uh oh im sorry I did find a relay behind the center of the dash, sorry