jerbear

Engine rebuilt 8 months ago, new clutch at installation. Truck has been driving great until this weekend. Shifts fine through all gears. No slipping or funny noises until I get cruising in fourth gear. Goes into gear fine, but if I go to give more gas while in fourth the RPM's jump up and there is no more acceleration. There are no other problems or symptoms at this time. Any ideas on what may be going wrong here?