clickdeath.05

My 88 pickup wont stay locked into 4 wheel high. The 4 low works great, but not high. I can move the lever into 4 high and as soon as i push the gas and start moving it kicks back into 2WD. I don't know if it's related, but the speedometer also quit working around the same time. I replaced the cable and cluster and it still doesn't work. Please help